Light emitting device and electronic equipment

ABSTRACT

A display device capable of keeping the luminance constant irrespective of temperature change is provided as well as a method of driving the display device. A current mirror circuit composed of transistors is placed in each pixel. A first transistor and a second transistor of the current mirror circuit are connected such that the drain current of the first transistor is kept in proportion to the drain current of the second transistor irrespective of the load resistance value. The drain current of the first transistor is controlled by a driving circuit in accordance with a video signal and the drain current of the second transistor is caused to flow into an OLED, thereby controlling the OLED drive current and the luminance of the OLED.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/966,379, filed Dec. 13, 2010, now allowed, which is a continuation ofU.S. application Ser. No. 10/902,421, filed Jul. 30, 2004, now U.S. Pat.No. 7,851,796, which is a continuation of U.S. application Ser. No.10/077,760, filed Feb. 20, 2002, now U.S. Pat. No. 6,777,710, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2001-050644 on Feb. 26, 2001, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an OLED (organic light emitting diode)panel obtained by forming an OLED on a substrate and sealing the OLEDbetween the substrate and a cover member. The invention also relates toan OLED module in which an IC including a controller, or the like, ismounted to the OLED panel. In this specification, ‘light emittingdevice’ is the generic term for the OLED panel and for the OLED module.Electronic equipment using the light emitting device is also included inthe present invention.

2. Description of the Related Art

Being self-luminous, OLEDs eliminate the need for a backlight that isnecessary in liquid crystal display devices (LCDs), and thus they aremost suitable when manufacturing thinner devices. Also, theself-luminous OLEDs are high in visibility and have no limit in terms ofviewing angle. These are the reasons for the attention that lightemitting devices using the OLEDs are receiving in recent years asdisplay devices to replace CRTs and LCDs.

An OLED has a layer containing an organic compound (organic lightemitting material) that provides luminescence (electroluminescence) whenan electric field is applied (the layer is hereinafter referred to asorganic light emitting layer), in addition to an anode layer and acathode layer. Luminescence obtained from organic compounds isclassified into light emission upon return to the base state fromsinglet excitation (fluorescence) and light emission upon return to thebase state from triplet excitation (phosphorescence). A light emittingdevice according to the present invention can use one or both types ofthe light emission.

In this specification, all the layers that are provided between an anodeand a cathode together make an organic light emitting layer.Specifically, the organic light emitting layer includes a light emittinglayer, a hole injection layer, an electron injection layer, a holetransporting layer, an electron transporting layer, etc. A basicstructure of an OLED is a laminate of an anode, a light emitting layer,and a cathode layered in this order. The basic structure can be modifiedinto a laminate of an anode, a hole injection layer, a light emittinglayer, and a cathode layered in this order, or a laminate of an anode, ahole injection layer, a light emitting layer, an electron transportinglayer, and a cathode layered in this order.

The problem in putting a light emitting device into practice is loweringin luminance of OLED which accompanies degradation of its organic lightemitting material.

Organic light emitting materials are weak against moisture, oxygen,light, and heat, which accelerate degradation of the organic lightemitting materials. Specifically, the rate of degradation of an organiclight emitting material depends on the structure of a device for drivingthe light emitting device, characteristics of the organic light emittingmaterial, materials of electrodes, manufacture process conditions, howthe light emitting device is driven, etc.

Even when the voltage applied to the organic light emitting layer isconstant, the luminance of the OLED is lowered as the organic lightemitting layer degrades, and an image displayed therefore becomesunclear. In this specification, a voltage applied to an organic lightemitting layer from a pair of electrodes is called an OLED drive voltage(Vel).

When an image is displayed in color by using three types of OLEDs thatrespectively emit red (R) light, green (G) light, and blue (B) light,different organic materials are used to form organic light emittinglayers of OLEDs of different colors. Accordingly, the rate ofdegradation of organic light emitting layer may vary between OLEDs ofdifferent colors. Then the difference in luminance between OLEDs ofdifferent colors will be noticeably large as time passes, making itimpossible for the light emitting device to display an image in desiredcolors.

The temperature of organic light emitting layer is influenced by theoutside temperature and heat generated from the OLED panel itself.Generally, the amount of current flowing in an OLED varies depending onthe temperature. FIG. 26 shows a change in voltage-currentcharacteristic of an OLED when the temperature of its organic lightemitting layer is changed. With the voltage kept constant, the OLEDdrive current is increased as the temperature of the organic lightemitting layer rises. Since the OLED drive current is in proportion tothe OLED luminance, the luminance of the OLED becomes higher as the OLEDdrive current becomes larger. Since a change in temperature of theorganic light emitting layer thus causes a change in OLED luminance,displaying an image in desired gray scales is difficult and currentconsumption of the light emitting device is increased accompanying atemperature rise.

Generally, temperature change brings varying degrees of changes in OLEDdrive current to different types of organic light emitting materialsand, therefore, in color display, the luminance could be changed bytemperature change differently for OLEDs of different colors. It isimpossible to obtain desired colors when OLEDs of different colors losetheir luminance balance.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a light emitting devicecapable of keeping the luminance constant and displaying an image indesired colors without being influenced by degradation of its organiclight emitting layer or by temperature change.

The present inventors have taken notice of the fact that the luminanceof OLED is lowered by degradation less when light is emitted with acurrent flow to an OLED kept constant than when light is emitted withthe OLED drive voltage kept constant. (In this specification, a currentflowing into an OLED is called an OLED drive current (Iel).) Then, thepresent inventors have thought of preventing a change in OLED luminancedue to degradation of OLED by controlling the OLED luminance withcurrent instead of voltage.

Specifically, a current mirror circuit composed of transistors isprovided in each pixel in the present invention. The current mirrorcircuit is used to control the OLED drive current. A first transistorand a second transistor of the current mirror circuit are connected suchthat the drain current of the first transistor is kept substantiallyequal to the drain current of the second transistor irrespective of theload resistance value.

A drain current I₁ of the first transistor is controlled by a signalline driving circuit. The amount of drain current I₁ of the firsttransistor is always equal to the amount of drain current I₂ of thesecond transistor irrespective of the load resistance value.Accordingly, the drain current I₂ of the second transistor is controlledby the signal line driving circuit.

The second transistor is connected to an OLED with a single or pluralcircuit elements interposed therebetween, so that the drain current I₂thereof flows into the OLED. Therefore the value of OLED drive currentflowing into the OLED is controlled by the signal line driving circuitirrespective of the load resistance value. In other words, the OLEDdrive current can be controlled to have a desired value without beinginfluenced by difference in characteristics of transistors ordegradation of OLED.

With the above structure, the present invention can prevent theluminance of OLED from lowering even when the organic light emittinglayer is degraded and therefore can display a clear image. If the lightemitting device is to display an image in color using OLEDs of differentcolors and the rate of degradation of organic light emitting layervaries between the OLEDs of different colors, the present invention iscapable of keeping the luminance of light of different colors balancedand display in desired colors.

Furthermore, the present invention can set the OLED drive current to adesired value despite a change in temperature of the organic lightemitting layer due to the outside temperature and heat generated fromthe OLED panel itself. Since the OLED drive current is in proportion tothe OLED luminance, the luminance of OLED can be prevented from changingand current consumption accompanying a temperature rise can be avoided.If the light emitting device is to display an image in color, theluminance of the OLEDs of different colors can be prevented fromchanging to keep the luminance of light of different colors balanced anddisplay in desired colors.

Generally, temperature change brings varying degrees of changes in OLEDdrive current to different types of organic light emitting materialsand, therefore, in color display, the luminance could be changed bytemperature change differently for OLEDs of different colors. However,the light emitting device of the present invention can obtain a desiredluminance irrespective of temperature change to thereby keep theluminance of light of different colors balanced. An image thus can bedisplayed in desired colors.

In a common light emitting device, the electric potential of a wiringline used to supply a current to pixels is slightly lowered as thewiring line becomes longer because of the resistance of the wiring lineitself. This electric potential is lowered to widely varying degreesdepending on an image to be displayed. When the ratio of higher grayscale pixels to all of the pixels that receive a current from the samewiring line is large, in particular, the current flowing through thewiring lines is increased in amount to make lowering of electricpotential noticeable. When the electric potential is lowered, a smallervoltage is applied to the OLED of each pixel to reduce the amount ofcurrent supplied to each pixel. Therefore, the amount of currentsupplied to one pixel is changed as well as the gray scale numberthereof when the gray scale number of other pixels that receive acurrent from the same wiring line as the one pixel is changed, making itimpossible for the one pixel to keep a constant gray scale. In the lightemitting device of the present invention, on the other hand, a measuredvalue and a reference value are obtained to correct the OLED currenteach time a new image is displayed. Therefore, a desired gray scalenumber is obtained for every new image through correction.

In the light emitting device of the present invention, a transistor usedin a pixel may be one formed from single crystal silicon or may be athin film transistor formed from polycrystalline silicon or amorphoussilicon.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a top view of a light emitting deviceof the present invention;

FIG. 2 is a circuit diagram of a pixel in a light emitting device of thepresent invention;

FIGS. 3A and 3B are timing charts of signals inputted to scanning lines;

FIGS. 4A and 4B are schematic diagrams of a pixel being driven:

FIG. 5 is a timing chart showing at which points writing periods anddisplay periods are started in an analog driving method;

FIG. 6 is a timing chart showing at which points writing periods anddisplay periods are started in a digital driving method;

FIG. 7 is a circuit diagram of a pixel in a light emitting device of thepresent invention;

FIG. 8 is a circuit diagram of a pixel in a light emitting device of thepresent invention;

FIGS. 9A to 9D are diagrams showing a method of manufacturing a lightemitting device according to the present invention;

FIGS. 10A to 10C are diagrams showing a method of manufacturing a lightemitting device according to the present invention;

FIGS. 11A and 11B are diagrams showing a method of manufacturing a lightemitting device according to the present invention;

FIG. 12 is a top view of a pixel in a light emitting device of thepresent invention;

FIG. 13 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIGS. 14A and 14B are diagrams showing a method of manufacturing a lightemitting device according to the present invention;

FIG. 15 is a top view of a pixel in a light emitting device of thepresent invention;

FIG. 16 is a top view of a pixel in a light emitting device of thepresent invention;

FIG. 17 is a block diagram of a signal line driving circuit:

FIG. 18 is a detailed diagram of a signal line driving circuit in adigital driving method;

FIG. 19 is a circuit diagram of a current setting circuit in a digitaldriving method;

FIG. 20 is a block diagram of a scanning line driving circuit;

FIG. 21 is a timing chart showing at which points writing periods anddisplay periods are started in a digital driving method;

FIG. 22 is a timing chart showing at which points writing periods anddisplay periods are started in a digital driving method;

FIG. 23 is a timing chart showing at which points writing periods anddisplay periods are started in a digital driving method;

FIGS. 24A to 24C are diagrams showing the exterior and sectional viewsof a light emitting device of the present invention;

FIGS. 25A to 25H are diagrams of electronic equipment using a lightemitting device of the present invention;

FIG. 26 is a graph showing the voltage-current characteristic of anOLED:

FIG. 27 is a sectional view of a pixel in a light emitting device of thepresent invention;

FIG. 28 is a top view of an element substrate in a light emitting deviceof the present invention;

FIG. 29 is an enlarged view of an element substrate in a light emittingdevice of the present invention;

FIGS. 30A to 30C are circuit diagrams of pixels in a light emittingdevice of the present invention; and

FIGS. 31A and 31B are detailed diagrams of a signal line driving circuitin a digital driving method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

FIG. 1 is a block diagram showing the structure of an OLED panel of thepresent invention. Reference symbol 100 denotes a pixel portion. Thepixel portion has a plurality of pixels 101 that form a matrix. Denotedby 102 and 103 are a signal line driving circuit and a scanning linedriving circuit, respectively.

In FIG. 1, the signal line driving circuit 102 and the scanning linedriving circuit 103 are formed on the same substrate on which the pixelportion 100 is formed. However, the present invention is not limited tothis structure. The signal line driving circuit 102 and the scanningline driving circuit 103 may be formed on a substrate that is connectedthrough an FPC or other connectors to a substrate on which the pixelportion 100 is formed. Although the panel in FIG. 1 has one signal linedriving circuit 102 and one scanning line driving circuit 103, thepresent invention is not limited thereto. The number of signal linedriving circuits and scanning line driving circuits to be provided isfreely determined by a designer.

In this specification, connection means electric connection.

In FIG. 1, the pixel portion 100 is provided with signal lines S1 to Sx,power supply lines V1 to Vx, and scanning lines G1 to Gy. The number ofsignal lines may not always match the number of power supply lines. Thepixel portion may have other wiring lines than these wiring lines.

The power supply lines V1 to Vx are kept at a given electric potential.Although shown in FIG. 1 is the structure of a light emitting device fordisplaying a monochromatic image, the present invention can be appliedto a light emitting device for displaying a color image. In this case,not all of the power supply lines V1 to Vx may be kept at the same levelof electric potential and power supply lines for one color may have adifferent level of electric potential than power supply lines foranother color.

FIG. 2 shows a detailed structure of the pixels 101 illustrated inFIG. 1. A pixel shown in FIG. 2 is one of the pixels 101. The pixel 101has a signal line Si (one of S1 to Sx), a scanning line Gj (one of G1 toGy), and a power supply line Vi (one of V1 to Vx).

Each of the pixels 101 has, at least, a transistor Tr1 first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), an OLED 104, and a storage capacitor 105.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the scanning line Gj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a drain region of the transistor Tr1. The transistor Tr5 has a sourceregion and a drain region one of which is connected to the signal lineSi and the other of which is connected to a gate electrode of thetransistor Tr3.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 104. The OLED 104 has an anode and a cathode. In thisspecification, the cathode is called an opposite electrode (secondelectrode) when the anode is used as a pixel electrode (first electrode)and, when the cathode serves as the pixel electrode, the anode is calledthe opposite electrode.

The electric potential of the power supply line Vi (power supplyelectric potential) is kept constant. The electric potential of theopposite electrode is also kept constant.

The transistor Tr4 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistor Tr5. However, thetransistor Tr4 and the transistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The storage capacitor 105 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 105is provided to maintain the voltage between the gate electrode of thetransistor Tr3 and the source region thereof (gate voltage) moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor s between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

Next, driving of the light emitting device of the present invention willbe described with reference to FIGS. 3A to 4B. The description ondriving of the light emitting device of the present invention can bedivided into a description for a writing period Ta and a description fora display period Td. FIGS. 3A and 3B are timing charts of scanninglines. A period in which a scanning line is selected, in other words, aperiod in which every transistor whose gate electrode is connected tothe selected scanning line is turned ON, is expressed as ON. On theother hand, a period in which a scanning line is not selected, in otherwords, a period in which every transistor whose gate electrode isconnected to the scanning line is turned OFF, is expressed as OFF. FIGS.4A and 4B schematically show connection of the transistor Tr1, thetransistor Tr2 and the transistor Tr3 during a writing period Ta and adisplay period Td.

In a writing period Ta, the scanning lines G1 to Gy are selected inorder as shown in FIG. 3A. Then a constant current Ic flows between thesignal lines S1 to Sx and the power supply lines V1 to Vx in accordancewith the electric potential of a video signal inputted to the signalline driving circuit 102. In this specification, the current Ic iscalled a signal current.

FIG. 4A shows a schematic diagram of one of the pixels 101 when theconstant current Ic flows in the signal line Si in a writing period Ta.Denoted by 106 is a terminal for connecting the pixel to a power supplyfor giving an electric potential to the opposite electrode. 107 denotesa constant current supply of the signal line driving circuit 102.

The transistors Tr4 and Tr5 are ON and, when the signal line Si receivesthe constant current Ic, the constant current Ic flows between the drainregion of the transistor Tr1 and the source region thereof. The amountof the constant current Ic is controlled by the constant current supply107 so that the transistor Tr1 operates in a saturation range. In thesaturation range, V_(GS) is given as the electric potential differencebetween the gate electrode and the source region (gate voltage), μ isgiven as the mobility of the transistor, C_(O) as the gate capacitanceper unit area, W/L as the ratio of a channel width W of the channelformation region to a channel length L thereof, V_(TH) as the threshold,and I₁ as the drain current of the transistor Tr1. Then the followingEquation 1 is obtained.I ₁ =μC _(O) W/L(V _(GS) −V _(TH))²/2  Equation 1

In Equation 1, μ, C_(O), W/L, and V_(TH) are values fixed for therespective transistors. The drain current I₁ of the transistor Tr1 isalso kept constant at Ic by the constant current supply 107.Accordingly, the gate voltage V_(GS) of the transistor Tr1 is determinedby the value of the current Ic as shown in Equation 1.

The gate electrode of the transistor Tr2 is connected to the gateelectrode of the transistor Tr1. The source region of the transistor Tr2is connected to the source region of the transistor Tr1. Therefore thegate voltage of the transistor Tr2 is equal to the gate voltage of thetransistor Tr1. Accordingly, a drain current I₂ of the transistor Tr2 iskept at the same level as the drain current of the transistor Tr1 tosatisfy I₂=Ic.

The drain current of the transistor Tr3 is equal to the drain current I₂of the transistor Tr2. Satisfying Equation 1, the transistor Tr3generates a gate voltage in an amount according to the drain current I₂.

The drain current I₂ of the transistor Tr2 thus flows into the OLED 104through a channel formation region of the transistor Tr3. Accordingly,the OLED drive current is equal to the constant current Ic set by theconstant current supply 107.

The OLED 104 emits light at a luminance according to the amount of OLEDdrive current. When the OLED drive current is extremely close to 0 orwhen the OLED drive current flows in the reverse bias direction, theOLED 104 does not emit light.

After all of the scanning lines G1 to Gy are selected and the aboveoperation is conducted on pixels in every line, the writing period Ta isended. As the writing period Ta is ended, a display period Td isstarted.

FIG. 3B is a timing chart of the scanning lines in the display periodTd. In the display period Td, none of the scanning lines G1 to Gy areselected.

FIG. 4B is a schematic diagram of a pixel in the display period Td. Thetransistors Tr4 and Tr5 are turned OFF. The source regions of thetransistor Tr1 and of the transistor Tr2 are connected to the powersupply line Vi and kept at a given electric potential (power supplyelectric potential).

In the display period Td, the drain region of the transistor Tr1 is in aso-called floating state in which Tr1 does not receive an electricpotential from other wiring lines nor from a power supply. On the otherhand, the transistors Tr2 and Tr3 maintain V_(GS) set in the writingperiod Ta, which means that the drain current I₂ of the transistor Tr2is still kept at Ic and that the transistor Tr3 remains ON. Thereforethe OLED drive current set in the writing period Ta is maintained duringthe display period Td and the OLED 104 emits light at a luminanceaccording to the amount of the OLED drive current.

In the case of a driving method using an analog video signal (analogdriving method), the amount of Ic is determined in accordance with ananalog video signal and the OLED 104 emits light at a luminanceaccording to the amount of Ic to obtain a gray scale. In this case, onewriting period Ta and one display period Td constitute one frame period,and one image is displayed in one frame period.

FIG. 5 is an example of the timing chart in an analog driving method.One frame period has y line periods, and one scanning line is selectedin each line period. Constant currents Ic (Ic1 to Icx) flow in signallines in each line period. In FIG. 5, Ic1[Lj] to Icx[Lj] representvalues of signal current flowing in the respective signal lines in aline period Lj (j=1 to y).

Starting points of the writing period Ta and the display period Td varybetween different lines and the starting point of a writing period forone line does not coincide with the starting point for another line.When the display period Td is completed for all of the pixels, one imageis displayed.

In the case of a time gray scale driving method using a digital videosignal (digital driving method), on the other hand, a writing period Taand a display period Td are repeatedly alternated in one frame period tomake it possible to display one image. If an image is displayed using nbit video signals, one frame period has at least n writing periods and ndisplay periods. The n writing periods (Ta1 to Tan) are respectivelyassociated with n bits of the n bit video signals, and so are the ndisplay periods (Td1 to Tdn).

FIG. 6 shows at which points the n writing periods (Ta1 to Tan) and then display periods (Td1 to Tdn) are started in one frame period. The axisof abscissa indicates time and the axis of ordinate indicates positionsof scanning lines of pixels.

A writing period Tam (m is an arbitrary number ranging from 1 to n) isfollowed by a display period associated with the same bit number as thewriting period Tam, in this case, a display period Tdm. One writingperiod Ta and one display period Td constitute one sub-frame period SF.A sub-frame period SFm consists of the wiring period Tam and the displayperiod Tdm that are associated with the m-th bit signal.

Lengths of sub-frame periods SF1 to SFn are set so as to satisfySF1:SF2: . . . :SFn=2⁰:2¹: . . . :2^(n-1).

A sub-frame period having a long display period may further be dividedto improve the quality of an image displayed. Specifics on how such asub-frame period is divided can be found in Japanese Patent ApplicationNo. 2000-267164.

The driving method shown in FIG. 6 obtains gray scale display bycontrolling the sum of lengths of display periods in one frame period inwhich a pixel emits light.

With the above structure, the present invention can prevent theluminance of OLED from lowering even when the organic light emittinglayer is degraded and therefore can display a clear image. If the lightemitting device is to display an image in color using OLEDs of differentcolors and the rate of degradation of organic light emitting layervaries between the OLEDs of different colors, the present invention iscapable of keeping the luminance of light of different colors balancedand display in desired colors.

Furthermore, the present invention can set the OLED drive current to adesired value despite a change in temperature of the organic lightemitting layer due to the outside temperature and heat generated fromthe OLED panel itself. Since the OLED drive current is in proportion tothe OLED luminance, the luminance of OLED can be prevented from changingand current consumption accompanying a temperature rise can be avoided.If the light emitting device is to display an image in color, theluminance of the OLEDs of different colors can be prevented fromchanging to keep the luminance of light of different colors balanced anddisplay in desired colors.

Generally, temperature change brings varying degrees of changes in OLEDdrive current in accordance with different types of organic lightemitting materials and, therefore, in color display, the luminance couldbe changed by temperature change differently for OLEDs of differentcolors. However, the light emitting device of the present invention canobtain a desired luminance irrespective of temperature change to therebykeep the luminance of light of different colors balanced. An image thuscan be displayed in desired colors.

In a common light emitting device, the electric potential of a wiringline used to supply a current to pixels is slightly lowered as thewiring line becomes longer because of the resistance of the wiring lineitself. This electric potential is lowered to widely varying degreesdepending on an image to be displayed. When the ratio of higher grayscale pixels to all of the pixels that receive a current from the samewiring line is large, in particular, the current flowing through thewiring lines is increased in amount to make lowering of electricpotential noticeable. When the electric potential is lowered, a smallervoltage is applied to the OLED of each pixel to reduce the amount ofcurrent supplied to each pixel. Therefore, the amount of currentsupplied to one given pixel is changed as well as the gray scale numberthereof when the gray scale number of other pixels that receive acurrent from the same wiring line as the one pixel is changed, making itimpossible for the one pixel to keep a constant gray scale. In the lightemitting device of the present invention, on the other hand, a measuredvalue and a reference value are obtained to correct the OLED currenteach time a new image is displayed. Therefore a desired gray scalenumber is obtained for every new image through correction.

Embodiment Mode 2

This embodiment mode describes a structure different from the one inFIG. 2 for the pixels 101 of FIG. 1.

The pixel structure of this embodiment mode is shown in FIG. 7. A pixelshown in FIG. 7 is one of the pixels 101. The pixel 101 has a signalline Si (one of S1 to Sx), a scanning line Gj (one of G1 to Gy), and apower supply line Vi (one of V1 to Vx).

Each of the pixels 101 has, at least, a transistor Tr1 (a first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), an OLED 104, and a storage capacitor 105.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the scanning line Gj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a drain region of the transistor Tr1. The transistor Tr5 has a sourceregion and a drain region one of which is connected to the drain regionof the transistor Tr1 and the other of which is connected to a gateelectrode of the transistor Tr3.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 104. The electric potential of the power supply line Vi(power supply electric potential) is kept constant. The electricpotential of the opposite electrode is also kept constant.

The transistor Tr4 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistor Tr5. However, thetransistor Tr4 and the transistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The storage capacitor 105 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 105is provided to maintain the gate voltage of the transistor Tr3 moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor s between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

As in the case of the pixel shown in FIG. 2, the description onoperation of a light emitting device that has the pixel of FIG. 7 can bedivided into a description for a writing period Ta and a description fora display period Td. The pixel in FIG. 7 operates the same way as thepixel in FIG. 2 in the writing period Ta and the display period Td.Therefore the descriptions given in Embodiment Mode 1 on FIGS. 3A to 4Bapply to the pixel in FIG. 7 and will not be repeated here.

Embodiment Mode 3

This embodiment mode describes a structure different from those in FIGS.2 and 7 for the pixels 101 of FIG. 1.

The pixel structure of this embodiment mode is shown in FIG. 8. A pixelshown in FIG. 8 is one of the pixels 101. The pixel 101 has a signalline Si (one of S1 to Sx), a scanning line Gj (one of G1 to Gy), and apower supply line Vi (one of V1 to Vx).

Each of the pixels 101 has, at least, a transistor Tr1 (a first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), an OLED 104, and a storage capacitor 105.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the scanning line Gj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a gate electrode of the transistor Tr3. The transistor Tr5 has asource region and a drain region one of which is connected to the gateelectrode of the transistor Tr3 and the other of which is connected to adrain region of the transistor Tr1.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 104. The electric potential of the power supply line Vi(power supply electric potential) is kept constant. The electricpotential of the opposite electrode is also kept constant.

The transistor Tr4 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistor Tr5. However, thetransistor Tr4 and the transistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The storage capacitor 105 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 105is provided to maintain the voltage between the gate electrode of thetransistor Tr3 and the source region thereof (gate voltage) moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor s between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

As in the case of the pixel shown in FIG. 2, the description onoperation of a light emitting device that has the pixel of FIG. 8 can bedivided into a description for a writing period Ta and a description fora display period Td. The pixel in FIG. 8 operates the same way as thepixel in FIG. 2 in the writing period Ta and the display period Td.Therefore the descriptions given in Embodiment Mode 1 on FIGS. 3A to 4Bapply to the pixel in FIG. 8 and will not be repeated here.

EMBODIMENTS

Embodiments of the present invention are described hereinafter.

Embodiment 1

Next, described with reference to FIGS. 9 to 13 is a method of formingthe light emitting device of the present invention. Here, the method ofsimultaneously forming, on the same substrate, transistors Tr2, Tr3 andTr5 of the pixel, and transistors of a driving portion providedsurrounding the pixel portion is described in detail according to steps.In addition, transistors Tr1 and Tr4 can be manufactured according tothe manufacturing method of transistors Tr2, Tr3, and Tr5. The pixelsshown in FIGS. 7, 8, 30A, 30B, and 30C can also be manufacturedaccording to the manufacturing method shown in this embodiment.

This embodiment uses a substrate 900 of a glass such as bariumborosilicate glass or aluminoborosilicate glass as represented by theglass #7059 or the glass #1737 of Corning Co. There is no limitation onthe substrate 900 provided it has a property of transmitting light, andthere may be used a quartz substrate. There may be further used aplastic substrate having heat resistance capable of withstanding thetreatment temperature of this embodiment.

Referring next to FIG. 9 (A), an underlying film 901 comprising aninsulating film such as silicon oxide film, silicon nitride film orsilicon oxynitride film is formed on the substrate 900. In thisembodiment, the underlying film 901 has a two-layer structure. There,however, may be employed a structure in which a single layer or two ormore layers are laminated on the insulating film. The first layer of theunderlying film 901 is a silicon oxynitride film 901 a formedmaintaining a thickness of from 10 to 200 nm (preferably, from 50 to 100nm) relying upon a plasma CVD method by using SiH₄, NH₃ and N₂O asreaction gases. In this embodiment, the silicon oxynitride film 901 a(having a composition ratio of Si=32%, 0=27%, N=24%, H=17%) is formedmaintaining a thickness of 50 nm. The second layer of the underlyingfilm 901 is a silicon oxynitride film 901 b formed maintaining athickness of from 50 to 200 nm (preferably, from 100 to 150 nm) relyingupon the plasma CVD method by using SiH₄ and N₂O as reaction gases. Inthis embodiment, the silicon oxynitride film 901 b (having a compositionratio of Si=32%, 0=59%, N=7%, H=2%) is formed maintaining a thickness of100 nm.

Then, semiconductor layers 902 to 905 are formed on the underlying film901. The semiconductor layers 902 to 905 are formed by forming asemiconductor film having an amorphous structure by a known means(sputtering method. LPCVD method or plasma CVD method) followed by aknown crystallization processing (laser crystallization method, heatcrystallization method or heat crystallization method using a catalystsuch as nickel), and patterning the crystalline semiconductor film thusobtained into a desired shape. The semiconductor layers 902 to 905 areformed in a thickness of from 25 to 80 nm (preferably, from 30 to 60nm). Though there is no limitation on the material of the crystallinesemiconductor film, there is preferably used silicon or asilicon-germanium (Si_(x)Ge_(1-N)(X=0.0001 to 0.02)) alloy. In thisembodiment, the amorphous silicon film is formed maintaining a thicknessof 55 nm relying on the plasma CVD method and, then, a solutioncontaining nickel is held on the amorphous silicon film. The amorphoussilicon film is dehydrogenated (500° C., one hour), heat-crystallized(550° C. four hours) and is, further, subjected to the laser annealingto improve the crystallization, thereby to form a crystalline siliconfilm. The crystalline silicon film is patterned by the photolithographicmethod to form semiconductor layers 902 to 905.

The semiconductor layers 902 to 905 that have been formed may further bedoped with trace amounts of an impurity element (boron or phosphorus) tocontrol the threshold value of the TFT.

In forming the crystalline semiconductor film by the lasercrystallization method, further, there may be employed an excimer laserof the pulse oscillation type or of the continuously light-emittingtype, a YAG laser or a YVO₄ laser. When these lasers are to be used, itis desired that a laser beam emitted from a laser oscillator is focusedinto a line through an optical system so as to fall on the semiconductorfilm. The conditions for crystallization are suitably selected by aperson who carries out the process. When the excimer laser is used, thepulse oscillation frequency is set to be 300 Hz and the laser energydensity to be from 100 to 400 mJ/cm² (typically, from 200 to 300mJ/cm²). When the YAG laser is used, the pulse oscillation frequency isset to be from 30 to 300 kHz by utilizing the second harmonics and thelaser energy density to be from 300 to 600 mJ/cm² (typically, from 350to 500 mJ/cm²). The whole surface of the substrate is irradiated withthe laser beam focused into a line of a width of 100 to 1000 μm, forexample, 400 μm, and the overlapping ratio of the linear beam at thismoment is set to be 50 to 90%.

Then, a gate insulating film 906 is formed to cover the semiconductorlayers 902 to 905. The gate insulating film 906 is formed of aninsulating film containing silicon maintaining a thickness of from 40 to150 nm by the plasma CVD method or the sputtering method. In thisembodiment, the gate insulating film is formed of a silicon oxynitridefilm (composition ratio of Si=32%, O=59%, N=7%, H=2%) maintaining athickness of 110 nm by the plasma CVD method. The gate insulating filmis not limited to the silicon oxynitride film but may have a structureon which is laminated a single layer or plural layers of an insulatingfilm containing silicon.

When the silicon oxide film is to be formed, TEOS (tetraethylorthosilicate) and O₂ are mixed together by the plasma CVD method, andare reacted together under a reaction pressure of 40 Pa, at a substratetemperature of from 300 to 400° C., at a frequency of 13.56 MHz and adischarge electric power density of from 0.5 to 0.8 W/cm². The thusformed silicon oxide film is, then, heat annealed at 400 to 500° C.thereby to obtain the gate insulating film having good properties.

Then, a heat resistant conductive layer 907 is formed on the gateinsulating film 906 maintaining a thickness of from 200 to 400 nm(preferably, from 250 to 350 nm) to form the gate electrode. Theheat-resistant conductive layer 907 may be formed as a single layer ormay, as required, be formed in a structure of laminated layers of plurallayers such as two layers or three layers. The heat resistant conductivelayer contains an element selected from Ta, Ti and W, or contains analloy of the above element, or an alloy of a combination of the aboveelements. The heat-resistant conductive layer is formed by thesputtering method or the CVD method, and should contain impurities at adecreased concentration to decrease the resistance and should,particularly, contain oxygen at a concentration of not higher than 30ppm. In this embodiment, the W film is formed maintaining a thickness of300 nm. The W film may be formed by the sputtering method by using W asa target, or may be formed by the hot CVD method by using tungstenhexafluoride (WF₆). In either case, it is necessary to decrease theresistance so that it can be used as the gate electrode. It is,therefore, desired that the W film has a resistivity of not larger than20 μΩcm. The resistance of the W film can be decreased by coarsening thecrystalline particles. When W contains much impurity elements such asoxygen, the crystallization is impaired and the resistance increases.When the sputtering method is employed, therefore, a W target having apurity of 99.9999% or 99.99% is used, and the W film is formed whilegiving a sufficient degree of attention so that the impurities will notbe infiltrated from the gaseous phase during the formation of the film,to realize the resistivity of from 9 to 20 μΩcm.

On the other hand, the Ta film that is used as the heat-resistantconductive layer 907 can similarly be formed by the sputtering method.The Ta film is formed by using Ar as a sputtering gas. Further, theaddition of suitable amounts of Xe and Kr into the gas during thesputtering makes it possible to relax the internal stress of the filmthat is formed and to prevent the film from being peeled off. The Tafilm of α-phase has a resistivity of about 20 μΩcm and can be used asthe gate electrode but the Ta film of β-phase has a resistivity of about180 μΩcm and is not suited for use as the gate electrode. The TaN filmhas a crystalline structure close to the α-phase. Therefore, if the TaNfilm is formed under the Ta film, there is easily formed the Ta film ofα-phase. Further, though not diagramed, formation of the silicon filmdoped with phosphorus (P) maintaining a thickness of about 2 to about 20nm under the heat resistant conductive layer 907 is effective infabricating the device. This helps improve the intimate adhesion of theconductive film formed thereon, prevent the oxidation, and prevent traceamounts of alkali metal elements contained in the heat resistantconductive layer 907 from being diffused into the gate insulating film906 of the first shape. In any way, it is desired that theheat-resistant conductive layer 907 has a resistivity over a range offrom 10 to 50 μΩcm.

Next, a mask 908 is formed by a resist relying upon thephotolithographic technology. Then, a first etching is executed. Thisembodiment uses an ICP etching device, uses Cl₂ and CF₄ as etchinggases, and forms a plasma with RF (13.56 MHz) electric power of 3.2W/cm² under a pressure of 1 Pa. The RF (13.56 MHz) electric power of 224mW/cm² is supplied to the side of the substrate (sample stage), too,whereby a substantially negative self bias voltage is applied. Underthis condition, the W film is etched at a rate of about 100 nm/min. Thefirst etching treatment is effected by estimating the time by which theW film is just etched relying upon this etching rate, and is conductedfor a period of time which is 20% longer than the estimated etchingtime.

The conductive layers 909 to 913 having a first tapered shape are formedby the first etching treatment. The conductive layers 909 to 913 aretapered at an angle of from 15 to 30°. To execute the etching withoutleaving residue, over-etching is conducted by increasing the etchingtime by about 10 to 20%. The selection ratio of the silicon oxynitridefilm (gate insulating film 906) to the W film is 2 to 4 (typically, 3).Due to the over etching, therefore, the surface where the siliconoxynitride film is exposed is etched by about 20 to about 50 nm (FIG. 9(B)).

Then, a first doping treatment is effected to add an impurity element ofa first type of electric conduction to the semiconductor layer. Here, astep is conducted to add an impurity element for imparting the n-type. Amask 908 forming the conductive layer of a first shape is left, and animpurity element is added by the ion-doping method to impart the n-typein a self-aligned manner with the conductive layers 909 to 913 having afirst tapered shape as masks. The dosage is set to be from 1×10¹³ to5×10¹⁴ atoms/cm²: so that the impurity element for imparting the n-typereaches the underlying semiconductor layer penetrating through thetapered portion and the gate insulating film 906 at the ends of the gateelectrode, and the acceleration voltage is selected to be from 80 to 160keV. As the impurity element for imparting the n-type, there is used anelement belonging to the Group 15 and, typically, phosphorus (P) orarsenic (As). Phosphorus (P) is used, here. Due to the ion-dopingmethod, an impurity element for imparting the n-type is added to thefirst impurity regions 914 to 917 over a concentration range of from1×10²⁰ to 1×10²¹ atoms/cm³ (FIG. 9 (C)).

In this step, the impurities turn down to the lower side of theconductive layers 909 to 913 of the first shape depending upon thedoping conditions, and it often happens that the first impurity regions914 to 917 are overlapped on the conductive layers 909 to 913 of thefirst shape.

Next, the second etching treatment is conducted as shown in FIG. 9 (D).The etching treatment, too, is conducted by using the ICP etchingdevice, using a mixed gas of CF₄ and Cl₂ as an etching gas, using an RFelectric power of 3.2 W·cm²: (13.56 MHz), a bias power of 45 mW/cm²(13.56 MHz) under a pressure of 1.0 Pa. Under this condition, there areformed the conductive layers 918 to 921 of a second shape. The endportions thereof are tapered, and the thickness gradually increases fromthe ends toward the inside. The rate of isotropic etching increases inproportion to a decrease in the bias electricity applied to the side ofthe substrate as compared to the first etching treatment, and the angleof the tapered portions becomes 30 to 60°. The mask 908 is etched at theedge by etching to form a mask 923. In the step of FIG. 9 (D), thesurface of the gate insulating film 906 is etched by about 40 nm.

Then, the doping is effected with an impurity element for imparting then-type under the condition of an increased acceleration voltage bydecreasing the dosage to be smaller than that of the first dopingtreatment. For example, the acceleration voltage is set to be from 70 to120 keV, the dosage is set to be 1×10¹³/cm² thereby to form firstimpurity regions 924 to 927 having an increased impurity concentration,and second impurity regions 928 to 931 that are in contact with thefirst impurity regions 924 to 927. In this step, the impurity may turndown to the lower side of the conductive layers 918 to 921 of the secondshape, and the second impurity regions 928 to 931 may be overlapped onthe conductive layers 918 to 922 of the second shape. The impurityconcentration in the second impurity regions is from 1×10¹⁶ to 1×10¹⁸atoms/cm³ (FIG. 10 (A)).

Referring to FIG. 10 (B), impurity regions 933 (933 a, 933 b) and 934(934 a, 934 b) of the conduction type opposite to the one conductiontype are formed in the semiconductor layers 902, 905 that form thep-channel TFTs. In this case, too, an impurity element for imparting thep-type is added using the conductive layers 918, 921 and 922 of thesecond shape as masks to form impurity regions in a self-aligned manner.At this moment, the semiconductor layers 903 and 904 forming then-channel TFTs are entirely covered for their surfaces by forming a mask932 of a resist. Here, the impurity regions 933 and 934 are formed bythe ion-doping method by using diborane (B₂H₆). The impurity element forimparting the p-type is added to the impurity regions 933 and 934 at aconcentration of from 2×10²⁰ to 2×10²¹ atoms/cm³.

If closely considered, however, the impurity regions 933, 934 can bedivided into two regions containing an impurity element that imparts then-type. Third impurity regions 933 a and 934 a contain the impurityelement that imparts the n-type at a concentration of from 1×10²⁰ to1×10²¹ atoms/cm³ and fourth impurity regions 933 b and 934 b contain theimpurity element that imparts the n-type at a concentration of from1×10¹⁷ to 1×10²⁰ atoms/cm³. In the impurity regions 933 b and 934 b,however, the impurity element for imparting the p-type is contained at aconcentration of not smaller than 1×10¹⁹ atoms/cm³ and in the thirdimpurity regions 933 a and 934 a. the impurity element for imparting thep-type is contained at a concentration which is 1.5 to 3 times as highas the concentration of the impurity element for imparting the n-type.Therefore, the third impurity regions work as source regions and drainregions of the p-channel TFTs without arousing any problem.

Referring next to FIG. 10 (C), a first interlayer insulating film 937 isformed on the conductive layers 918 to 921 of the second shape and onthe gate insulating film 906. The first interlayer insulating film 937may be formed of a silicon oxide film, a silicon oxynitride film, asilicon nitride film, or a laminated layer film of a combinationthereof. In any case, the first interlayer insulating film 937 is formedof an inorganic insulating material. The first interlayer insulatingfilm 937 has a thickness of 100 to 200 nm. When the silicon oxide filmis used as the first interlayer insulating film 937. TEOS and O₂ aremixed together by the plasma CVD method, and are reacted together undera pressure of 40 Pa at a substrate temperature of 300 to 400° C. whiledischarging the electric power at a high frequency (13.56 MHz) and at apower density of 0.5 to 0.8 W/cm². When the silicon oxynitride film isused as the first interlayer insulating film 937, this siliconoxynitride film may be formed from SiH₄, N₂O and NH₃, or from SiH₄ andN₂O by the plasma CVD method. The conditions of formation in this caseare a reaction pressure of from 20 to 200 Pa, a substrate temperature offrom 300 to 400° C. and a high-frequency (60 MHz) power density of from0.1 to 1.0 W/cm². As the first interlayer insulating film 937, further,there may be used a hydrogenated silicon oxynitride film formed by usingSiH₄, N₂O and H₂. The silicon nitride film, too, can similarly be formedby using SiH₄ and NH₃ by the plasma CVD method.

Then, a step is conducted for activating the impurity elements thatimpart the n-type and the p-type added at their respectiveconcentrations. This step is conducted by thermal annealing method usingan annealing furnace. There can be further employed a laser annealingmethod or a rapid thermal annealing method (RTA method). The thermalannealing method is conducted in a nitrogen atmosphere containing oxygenat a concentration of not higher than 1 ppm and, preferably, not higherthan 0.1 ppm at from 400 to 700° C. and, typically, at from 500 to 600°C. In this embodiment, the heat treatment is conducted at 550° C. for 4hours. When a plastic substrate having a low heat resistance temperatureis used as the substrate 900, it is desired to employ the laserannealing method.

Following the step of activation, the atmospheric gas is changed, andthe heat treatment is conducted in an atmosphere containing 3 to 100% ofhydrogen at from 300 to 450° C. for from 1 to 12 hours to hydrogenatethe semiconductor layer. This step is to terminate the dangling bonds of10¹⁶ to 10¹⁸/cm³ in the semiconductor layer with hydrogen that isthermally excited. As another means of hydrogenation, the plasmahydrogenation may be executed (using hydrogen excited with plasma). Inany way, it is desired that the defect density in the semiconductorlayers 902 to 905 be suppressed to be not larger than 10¹⁶/cm³. For thispurpose, hydrogen may be added in an amount of from 0.01 to 0.1 atomic%.

Then, a second interlayer insulating film 939 of an organic insulatingmaterial is formed maintaining an average thickness of from 1.0 to 2.0μm. As the organic resin material, there can be used polyimide, acrylic,polyamide, polyimideamide, or BCB (benzocyclobutene). When there isused, for example, a polyimide of the type that is heat polymerizedafter being applied onto the substrate, the second interlayer insulatingfilm is formed being fired in a clean oven at 300° C. When there is usedan acrylic, there is used the one of the two-can type. Namely, the mainmaterial and a curing agent are mixed together, applied onto the wholesurface of the substrate by using a spinner, pre-heated by using a hotplate at 80° C. for 60 seconds, and are fired at 250° C. for 60 minutesin a clean oven to form the second interlayer insulating film.

Thus, the second interlayer insulating film 939 is formed by using anorganic insulating material featuring good and flattened surface.Further, the organic resin material, in general, has a small dielectricconstant and lowers the parasitic capacitance. The organic resinmaterial, however, is hygroscopic and is not suited as a protectionfilm. It is, therefore, desired that the second interlayer insulatingfilm is used in combination with the silicon oxide film, siliconoxynitride film or silicon nitride film formed as the first interlayerinsulating film 937.

Thereafter, the resist mask of a predetermined pattern is formed, andcontact holes are formed in the semiconductor layers to reach theimpurity regions serving as source regions or drain regions. The contactholes are formed by dry etching. In this case, a mixed gas of CF₄, O₂and He is used as the etching gas, and the second interlayer insulatingfilm 939 of the organic resin material is etched. Thereafter. CF₄ and O₂are used as the etching gas to etch the first interlayer insulating film937. In order to further enhance the selection ratio relative to thesemiconductor layer. CHF; is used as the etching gas to etch the gateinsulating film 906 of the third shape, thereby to form the contactholes.

Here, the conductive metal film is formed by sputtering and vacuumvaporization and is patterned by using a mask and is, then, etched toform connecting wirings 940 to 947. Further, though not diagramed inthis embodiment, the wiring is formed by a laminate of a 50 nm thick Tifilm and a 500 nm thick alloy film (alloy film of Al and Ti).

Then, a transparent conductive film is formed thereon maintaining athickness of 80 to 120 nm, and is patterned to form a pixel electrode948 (FIG. 11 (A)). Therefore, the pixel electrode 948 is formed by usingan indium oxide-tin (ITO) film as a transparent electrode or atransparent conductive film obtained by mixing 2 to 20% of a zinc oxide(ZnO) into indium oxide.

Further, the pixel electrode 948 is formed being in contact with, andoverlapped on, the connecting wiring 946 that is electrically connectedto the drain region of the transistor Tr3.

FIG. 12 is a top view of the pixel after the process up through the stepof FIG. 11A is finished. The insulating film and the interlayerinsulating film are omitted from FIG. 12 in order to show positions ofthe wiring lines and of the semiconductor layers clearly. A sectionalview taken along the line A-A′ in FIG. 12 corresponds to the areaindicated by A-A in FIG. 11A.

FIG. 13 is a sectional view taken along the line B-B′ in FIG. 12. Thetransistor Tr4 has a gate electrode 975 that is a part of a scanningline 974. The gate electrode 975 is connected to a gate electrode 920 ofthe transistor Try. The semiconductor layer of the transistor Tr4 havetwo impurity regions 977 one of which is connected to the connectionwiring line 942 that functions as the signal line Si and the other ofwhich is connected to a connection wiring line 971.

The transistor Tr1 has a gate electrode 976, which is connected to agate electrode 922 of the transistor Tr2. The semiconductor layer of thetransistor Tr1 have two impurity regions 978 one of which is connectedto the connection wiring line 971 and the other of which is connected tothe connection wiring line 947 that functions as the power supply lineVi.

The connection wiring line 943 is connected to the impurity region 934 acommon to the transistors Tr2 and Tr3, and is connected to the gateelectrode 922 of the transistor Tr2.

Denoted by 970 is a storage capacitor, which has a semiconductor layer972, the gate insulating film 906, and a capacitance wiring line 973.One of impurity regions 979 in the semiconductor layer 972 is connectedto the connection wiring line 947 that functions as a power supply line.

Next, a third interlayer insulating film 949 having opening at aposition that coincides with the pixel electrode 948 is formed as shownin FIG. 11B. The third interlayer insulating film 949 is capable ofinsulating, and functions as a bank to separate organic light emittinglayers of adjacent pixels from each other. This embodiment uses a resistto form the third interlayer insulating film 949.

The third interlayer insulating film 949 in this embodiment has athickness of about 1 μm. The opening has a so-called reverse taper shapewhose width increases as the distance from the pixel electrode 948 isclosed. The reverse taper shape is obtained by covering the resist filmexcept the portion where the opening is to be formed, irradiating thefilm with UV light, and then removing the exposed portion with adeveloper.

By shaping the third interlayer insulating film 949 into a reverse tapershape as in this embodiment, organic light emitting layers of adjacentpixels are separated from each other when forming the organic lightemitting layers in a later step. Therefore cracking or peeling oforganic light emitting layers can be prevented even when the organiclight emitting layers and the third interlayer insulating film 949 havedifferent coefficient of thermal expansion.

Although a resist is used for the third interlayer insulating film inthis embodiment, polyimide, polyamide, acrylic, BCB (benzocyclobutene),or silicon oxide may be used instead in some cases. The third interlayerinsulating film 949 may be an organic or inorganic material as long asit is capable of insulating.

An organic light emitting layer 950 is formed next by evaporation. Thena cathode (MgAg electrode) 951 and a protective electrode 952 are formedby evaporation. It is desirable to remove moisture completely from thepixel electrode 948 by subjecting the pixel electrode to heat treatmentprior to forming the organic light emitting layer 950 and the cathode951. This embodiment uses a MgAg electrode as the cathode of the OLEDbut the cathode may be formed from other known materials.

A known material can be used for the organic light emitting layer 950.In this embodiment, the organic light emitting layer has a two-layerstructure consisting of a hole transporting layer and a light emittinglayer. The organic light emitting layer may additionally have one ormore layers out of a hole injection layer, an electron injection layer,and an electron transporting layer. Various combinations have beenreported and the organic light emitting layer of this embodiment cantake any of those.

The hole transporting layer of this embodiment is formed by evaporationfrom polyphenylene vinylene. The light emitting layer of this embodimentis formed by evaporation from polyvinyl carbazole with 30 to 40% of PBD,that is a 1, 3, 4-oxadiazole derivative, being molecule-dispersed. Thelight emitting layer is doped with about 1% of Coumarin 6 as greenluminescent center.

The protective electrode 952 alone can protect the organic lightemitting layer 950 from moisture and oxygen, but it is more desirable toadd a protective film 953. This embodiment uses a silicon nitride filmwith a thickness of 300 nm as the protective film 953. The protectivefilm and the protective electrode 952 may be formed in successionwithout exposing the device to the air.

The protective electrode 952 also prevents degradation of the cathode951. A typical material of the protective electrode is a metal filmmainly containing aluminum. Other materials may of course be used. Sincethe organic light emitting layer 950 and the cathode 91 are extremelyweak against moisture, the organic light emitting layer, the cathode,and the protective electrode 952 are desirably formed in successionwithout exposing them to the air. The organic light emitting layer andthe cathode are thus protected from the outside air.

The organic light emitting layer 950 is 10 to 400 nm in thickness(typically 60 to 150 nm), and the cathode 951 is 80 to 200 nm inthickness (typically MO to 150 nm).

Thus completed is a light emitting device structured as shown in FIG.11B. An area 954 where the pixel electrode 948, the organic lightemitting layer 950, and the cathode 951 overlap corresponds to the OLED.

A p-channel TFT 960 and an n-channel TFT 961 are TFTs of the drivingcircuit and constitute a CMOS circuit. The transistor Tr2 and thetransistor Tr5 are TFTs of the pixel portion. The TFTs of the drivingcircuit and the TFTs of the pixel portion can be formed on the samesubstrate.

In the case of a light emitting device using an OLED, its drivingcircuit can be operated by a power supply having a voltage of 5 to 6 V,10 V, at most. Therefore degradation of TFTs due to hot electron is nota serious problem. Also, smaller gate capacitance is preferred for theTFTs since the driving circuit needs to operate at high speed.Accordingly, in a driving circuit of a light emitting device using anOLED as in this embodiment, the second impurity region 929 and thefourth impurity region 933 b of the semiconductor layers of the TFTspreferably do not overlap with the gate electrode 918 and the gateelectrode 919, respectively.

The method of manufacturing the light emitting device of the presentinvention is not limited to the one described in this embodiment. Thelight emitting device of the present invention can be manufactured by aknown method.

Embodiment 2

In this embodiment, a method of manufacturing a light emitting devicedifferent from that in Embodiment 1 is described.

The process through the formation of the second interlayer insulatingfilm 939 is the same as in Embodiment 1. As shown in FIG. 14 (A), afterthe second interlayer insulating film 939 is formed, a passivation film981 is formed to contact the second interlayer insulating film 939.

The passivation film 981 is effective in preventing moisture containedin the second interlayer insulating film 939 from permeating the organiclight emitting layer 950 through the pixel electrode 948 or a thirdinterlayer insulating film 982. In the case where the second interlayerinsulating film 939 includes an organic resin material, it isparticularly effective to provide the passivation film 981 since theorganic resin material contains a large amount of moisture.

In this embodiment, a silicon nitride film is used as the passivationfilm 981.

Thereafter, a resist mask having a predetermined pattern is formed, andcontact holes reaching impurity regions, which are source regions ordrain regions, are formed in the respective semiconductor layers. Thecontact holes are formed by a dry etching method. In this case, thepassivation film 981 is first etched by using a gas mixture of the CF₄and O₂ as an etching gas, and then second interlayer insulating film 939comprised of the organic resin material is etched by using a gas mixtureof CF₄, O₂ and He as an etching gas. Subsequently, the first interlayerinsulating film 937 is etched with CF₄ and O₂ as an etching gas.Further, in order to raise a selection ratio with the semiconductorlayer, the etching gas is changed to CHF₃ to etch the third shape gateinsulating film 906, whereby the contact holes can be formed.

Then, a conductive metal film is formed by a sputtering method or avacuum evaporation method, patterning is performed with a mask, andthereafter, etching is performed. Thus, the connecting wirings 940 to947 are formed. Although not shown, the wirings are formed of alamination film of a Ti film with a thickness of 50 nm and an alloy filmwith a thickness of 500 nm (alloy film of Al and Ti) in this embodiment.

Subsequently, a transparent conductive film is formed thereon with athickness of 80 to 120 nm, and the pixel electrode 948 is formed bypatterning (FIG. 14 (A)). Note that an indium-tin oxide (ITO) film or atransparent conductive film in which indium oxide is mixed with 2 to 20%of zinc oxide (ZnO) is used for a transparent electrode in thisembodiment.

Further, the pixel electrode 948 is formed so as to contact and overlapthe connecting wiring 946. Thus, electrical connection between the pixelelectrode 948 and the drain region of the transistor Tr2 is formed.

Next, as shown in FIG. 14 (B), the third interlayer insulating, film 982having an opening portion at the position corresponding to the pixelelectrode 948 is formed. In this embodiment, side walls having a taperedshape are formed by using a wet etching method in forming the openingportion. Differently from the case shown in Embodiment 1. the organiclight emitting layer formed on the third interlayer insulating film 982is not separated. Thus, the deterioration of the organic light emittinglayer which derives from a step becomes a conspicuous problem if theside walls of the opening portion are not sufficiently gentle, whichrequires attention.

Note that although a film made of silicon oxide is used as the thirdinterlayer insulating film 982 in this embodiment, an organic resin filmsuch as polyimide, polyimide, acrylic or BCB (benzocyclobutene) may alsobe used depending on circumstances.

Then, it is preferable that, before the organic light emitting layer 950is formed on the third interlayer insulating film 982, plasma processingusing argon is conducted to the surface of the third interlayerinsulating film 982 to make close the surface of the third interlayerinsulating film 982. With the above structure, it is possible to preventmoisture from permeating the organic light emitting layer 950 from thethird interlayer insulating film 982.

Next, the organic light emitting layer 950 is formed by an evaporationmethod, and further, the cathode (MgAg electrode) 951 and the protectingelectrode 952 are formed by the evaporation method. At this time, it isdesirable that heat treatment is conducted to the pixel electrode 948 tocompletely remove moisture prior to the formation of the organic lightemitting layer 950 and the cathode 951. Note that, the MgAg electrode isused as the cathode of the OLED in this embodiment, but other knownmaterials may also be used.

Note that a known material can be used for the organic light emittinglayer 950. In this embodiment, the organic light emitting layer takes atwo-layer structure constituted of a hole transporting layer and a lightemitting layer. However, there may be a case where any one of a holeinjecting layer, an electron injecting layer and an electrontransporting layer is included in the organic light emitting layer.Various examples of combinations have been reported as described above,and any structure among those may be used.

In this embodiment, polyphenylene vinylene is formed by the evaporationmethod for forming the hole transporting layer. Further,polyvinylcarbazole dispersed with PBD of 1, 3, 4-oxadiazole derivativewith 30 to 40% molecules is formed by the evaporation method for formingthe light emitting layer, and about 1% of coumarin 6 is added thereto asthe emission center of green color.

Further, it is possible to protect the organic light emitting layer 950from moisture and oxygen in the protecting electrode 952, but theprotective film 953 may be, more preferably, provided. In thisembodiment, a silicon nitride film with a thickness of 300 nm isprovided as the protective film 953. This protective film may becontinuously formed without exposure to an atmosphere after theformation of the protecting electrode 952.

Moreover, the protecting electrode 952 is provided for preventingdeterioration of the cathode 951 and is typified by a metal filmcontaining aluminum as its main constituent. Of course, other materialsmay also be used. Further, since the organic light emitting layer 950and the cathode 951 are extremely easily affected by moisture, it isdesirable that the formation is continuously performed through theformation of the protecting electrode 952 without exposure to anatmosphere to thereby protect the organic light emitting layer againstan outer atmosphere.

Note that the thickness of the organic light emitting layer 950 may be10 to 400 nm (typically, 60 to 150 nm) and the thickness of the cathode951 may be 80 to 200 nm (typically, 100 to 150 nm).

Thus, the light emitting device with the structure as shown in FIG. 14(B) is completed. Note that the portion 954, where the pixel electrode948, the organic light emitting layer 950 and the cathode 951 areoverlapped one another, corresponds to the OLED.

The p-channel TFT 960 and the n-channel TFT 961 are the TFTs of thedriver circuit, and form a CMOS. The transistor Tr2, Tr3 and Tr5 are theTFTs of the pixel portion. The TFTs of the driver circuit and the TFTsof the pixel portion can be formed on the same substrate.

The method of manufacturing the light emitting device of the presentinvention is not limited to the manufacturing method described in thisembodiment. The light emitting device of the present invention can bemanufactured by using a known method.

Embodiment 3

This embodiment gives a description on the top view of the pixel shownin FIG. 7. FIG. 15 shows a top view of a pixel of this embodiment.Insulating films such as a insulating gate film and an interlayerinsulating film are omitted from FIG. 15 in order to show positions ofwiring lines and of semiconductor layers clearly. In FIG. 15, wiringlines formed on the same layer are similarly hatched. The top view inFIG. 15 is of the pixel after a pixel electrode is formed and before anorganic light emitting layer is formed.

The pixel shown in FIG. 15 has one scanning line 211, one signal line210, and one power supply line 217. Portions of the scanning line 211are denoted by 212 and 213 and respectively serve as gate electrodes oftransistors Tr4 and Tr5.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line 210 and the other of which is connectedto a drain region of a transistor Tr1 through a connection wiring line215. The transistor Tr5 has a source region and a drain region one ofwhich is connected to the drain region of the transistor Tr1 through theconnection wiring line 215 and the other of which is connected to acapacitance wiring line 216 through a connection wiring line 214.

A gate electrode 219 of the transistor Tr1 and a gate electrode 220 of atransistor Tr2 are connected to each other. The gate electrodes 219 and220 of the transistors Tr1 and Tr2 are connected to a drain region ofthe transistor Tr2 through a connection wiring line 221.

A source region of the transistor Tr1 is connected to the power supplyline 217. A source region of the transistor Tr2 is connected to thepower supply line 217.

A portion of the capacitance wiring line 216 is denoted by 218 andserves as a gate electrode of a transistor Tr3. The transistor Tr3 has asource region and a drain region one of which is connected to the drainregion of the transistor Tr2 and the other of which is connected to apixel electrode 223 through a connection wiring line 222.

Denoted by 224 is an active layer for forming a storage capacitor. Abovethe active layer 224 for forming a storage capacitor, the capacitancewiring line 216 is formed with a gate insulating film (not shown in thedrawing) interposed therebetween. An area where the capacitance wiringline 216 overlaps the gate insulating film and the active layer 224 forforming a storage capacitor corresponds to a storage capacitor 205.Above the capacitance wiring line 216, the power supply line 217 isformed with an interlayer insulating film (not shown) interposedtherebetween. Alternatively, a capacitor formed in an area where thecapacitance wiring line 216, the interlayer insulating film, and thepower supply line 217 overlap may be used as the storage capacitor 205.

Partitioning walls (banks) for separating pixels from one another areformed on the power supply line 217. This makes it possible to obtain astorage capacitor and a power supply line without reducing the apertureratio.

The top view of the pixel in this embodiment merely shows an example ofthe structure of the present invention, and this embodiment does notlimit the top view structure of the pixel shown in FIG. 7. Thisembodiment may be combined freely with Embodiment 1 or 2.

Embodiment 4

This embodiment gives a description on the top view of the pixel shownin FIG. 8. FIG. 16 shows a top view of a pixel of this embodiment.Insulating films such as an insulating film and an interlayer insulatingfilm are omitted from FIG. 16 in order to show positions of wiring linesand of semiconductor layers clearly. In FIG. 16, wiring lines formed onthe same layer are similarly hatched. The top view in FIG. 16 is of thepixel after a pixel electrode is formed and before an organic lightemitting layer is formed.

The pixel shown in FIG. 16 has one scanning line 311, one signal line310, and one power supply line 317. Portions of the scanning line 311are denoted by 312 and 313 and respectively serve as gate electrodes oftransistors Tr4 and Tr5.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line 310 and the other of which is connectedto a capacitance wiring line 316 through a connection wiring line 315.The transistor Tr5 has a source region and a drain region one of whichis connected to a drain region of the transistor Tr1 through aconnection wiring line 314 and the other of which is connected to thecapacitance wiring line 316 through the connection wiring line 315.

A gate electrode 319 of the transistor Tr1 and a gate electrode 320 of atransistor Tr2 are connected to each other. The gate electrodes 319 and320 of the transistors Tr1 and Tr2 are connected to a drain region ofthe transistor Tr2 through a connection wiring line 321.

A source region of the transistor Tr1 is connected to the power supplyline 317. A source region of the transistor Tr2 is connected to thepower supply line 317.

A portion of the capacitance wiring line 316 is denoted by 318 andserves as a gate electrode of a transistor Tr3. The transistor Tr3 has asource region and a drain region one of which is connected to the drainregion of the transistor Tr2 and the other of which is connected to apixel electrode 323 through a connection wiring line 322.

Denoted by 324 is an active layer for forming a storage capacitor. Abovethe active layer 324 for forming a storage capacitor, the capacitancewiring line 316 is formed with a gate insulating film (not shown in thedrawing) interposed therebetween. An area where the capacitance wiringline 316 overlaps the gate insulating film and the active layer 324 forforming a storage capacitor corresponds to a storage capacitor 305.Above the capacitance wiring line 316, the power supply line 317 isformed with an interlayer insulating film (not shown) interposedtherebetween. Alternatively, a capacitor formed in an area where thecapacitance wiring line 316, the interlayer insulating film, and thepower supply line 317 overlap may be used as the storage capacitor 305.

The top view of the pixel in this embodiment merely shows an example ofthe structure of the present invention, and this embodiment does notlimit the top view structure of the pixel shown in FIG. 8. Thisembodiment may be combined freely with Embodiment 1 or 2.

Embodiment 5

This embodiment describes a light emitting device having a structuredifferent from the one in Embodiment 1.

FIG. 27 is a sectional view of a pixel portion in the light emittingdevice of this embodiment. The light emitting device shown in FIG. 27has a red color pixel (R pixel) 800 r, a green color pixel (G pixel) 800g, and a blue color pixel (B pixel) 800 b. The structure of thisembodiment is applicable to a light emitting device for displaying amonochromatic image as well as a light emitting device for colordisplay.

Each of the pixels has transistors Tr2 and Tr3 formed on a substrate830. In a light emitting device of the present invention, each pixel hasat least transistors Tr1, Tr2. Tr3, Tr4, and Tr5. However, thetransistor Tr2 and Tr3 are shown in FIG. 27.

Pixel electrodes 802 r, 802 g, and 802 b (pixel electrodes 802) areconnected, through contact holes formed in a gate insulating film 811and an interlayer insulating film 807, to drain regions 809 r, 809 g,and 809 b of the third transistors Tr3, respectively.

The pixel electrodes are cathodes in this embodiment and do not transmitlight. MgAg electrodes are used as the cathodes of the OLEDs in thisembodiment but other known materials may be used instead.

Then an interlayer insulating film 805 having an opening 850 is formedto cover the pixel electrodes 802 r, 802 g, and 802 b and the interlayerinsulating film 807. The opening 850 are positioned so as to overlap thepixel electrodes 802 r, 802 g, and 802 b. Although a silicon oxide filmis used as the interlayer insulating film 805 in this embodiment,organic resin films formed from polyimide, polyimide, acrylic, or BCB(benzocyclobutene) may be used in some cases.

Organic light emitting layers 803 r, 803 g, and 803 b (organic lightemitting layers 803) are formed in the opening of the interlayerinsulating film 805 so as to come into contact with the pixel electrodes802 r, 802 g, and 802 b. respectively. The organic light emitting layers803 r, 803 g, and 803 b are separately formed by evaporation using ametal mask in the order in accordance with each color. Duringevaporation, materials of the organic light emitting layers 303 r, 803g, and 803 b may overflow or run from the opening. So try to contain theorganic light emitting layers in the opening of the third interlayerinsulating film 805.

Next, metal-containing conductive layers 806 are formed by evaporationon the interlayer insulating film 805 except the opening. The materialof the conductive layers 806 is desirably a low resistant metal. Theconductive layers 806 may be a laminate of plural conductive layers. Thematerial used for the conductive layers 806 is copper in this embodimentbut it is not limited thereto. Any known metal material can be used aslong as it has a resistance lower than that of the material of oppositeelectrodes. The conductive layers 806 help to reduce the resistance ofthe opposite electrodes to be formed later and, therefore, thisembodiment is suitable for a large-sized substrate.

Opposite electrodes 804 are formed next of a transparent conductive filmto cover the organic light emitting layers 803 r, 803 g, and 803 b andthe conductive layers 806. The transparent conductive film used in thisembodiment is an ITO film. The ITO film can be formed by evaporation.This embodiment describes particularly the case of using ion plating toform the ITO film.

Ion plating is one of vapor surface treatment techniques classified asevaporation. In ion plating, an evaporation material evaporated in oneway or other is ionized or excited by high frequency plasma or vacuumelectric discharge. The resultant ions are accelerated by giving anegative electric potential to the substrate to which the material is tobe deposited, so that the ions are adhered to the substrate.

Specific conditions for forming the opposite electrodes by ion platinginclude setting the pressure at 0.01 to 1 Pa in an inert gas atmosphereand keeping the substrate temperature at 100 to 300° C. duringevaporation. Desirably, ITO as an evaporation source has a sintereddensity of 70% or higher. The optimal conditions for ion plating can beset suitably by an operator.

When an evaporation material is ionized or excited by high frequencyplasma, the ionization rate or excitation rate of the evaporationmaterial is enhanced and, since the ionized or excited evaporationmaterial is in a high energy state, the ions are fully coupled withoxygen while keeping the evaporation rate high. Accordingly, a qualityfilm can be obtained quickly.

The opposite electrodes 804 in this embodiment are formed from atransparent conductive film by the ion plating described above to athickness of 80 to 120 nm. In this embodiment, the transparentelectrodes are formed from an indium tin oxide (ITO) film or atransparent conductive film obtained by mixing 2 to 20% of zinc oxide(ZnO) with indium oxide.

The method of forming the opposite electrodes of this embodiment is notlimited to the ion plating described above. However, an ITO film formedby ion plating adheres well and has high crystallinity at a relativelylow temperature, reducing the resistance thereof. Furthermore, ionplating can form a uniform film over a relatively large area andtherefore is suitable for a large-sized substrate.

An R OLED 801 r, a G OLED 801 g, and a B OLED 801 b are thus completedin each pixel. The OLEDs respectively have the pixel electrodes 802 r,802 g, and 802 b, the organic light emitting layers 803 r, 803 g, and803 b, and the opposite electrodes 804.

FIG. 28 is a top view of a substrate (element substrate) on which thetransistors of this embodiment are formed. In FIG. 28, a pixel portion831, a scanning line driving circuit 832, a signal line driving circuit833, and a terminal 834 are formed on a substrate 830. The drivingcircuits as well as power supply lines and opposite electrodes formed inthe pixel portion are connected to the terminal 834 through lead-outwiring lines 835.

If necessary, an IC chip on which a CPU, a memory and the like areformed may be mounted to the element substrate by COG (chip on glass).

The OLEDs are formed between the conductive layers 806 and the structurethereof is shown in FIG. 29. The pixel electrodes 802 are electrodes ofthe pixels and formed between the conductive layers 806. In the layerabove the pixel electrodes, the organic compound layers 803 are formedbetween the conductive layers 806. The organic compound layers 803successively form a stripe pattern over the plural pixel electrodes 802.

The opposite electrodes 804 are formed in the layer above the organiccompound layers 803 and the conductive layers 806 to come into contactwith the conductive layers 806.

The lead-out wiring lines 835 are formed in the same layer as scanninglines (not shown) and are not in direct contact with the conductivelayers 806. The contact is formed between the lead-out wiring lines 835and the opposite electrodes 804 at points where they overlap.

This embodiment may be combined freely with Embodiment 3 or 4.

Embodiment 6

This embodiment describes structures of driving circuits (a signal linedriving circuit and a scanning line driving circuit) in a light emittingdevice of the present invention which is driven by a digital drivingmethod.

FIG. 17 is a block diagram showing the structure of a signal linedriving circuit 601. Reference symbol 602 denotes a shift register, 603,a memory circuit A, 604, a memory circuit B, and 605, a constant currentcircuit.

Clock signals CLK and start pulse signals SP are inputted to the shiftregister 602. Digital video signals are inputted to the memory circuit A603 and latch signals are inputted to the memory circuit B 604. Theconstant current circuit 605 outputs a constant signal current Ic, whichis inputted to signal lines.

FIG. 18 shows a more detailed structure of the signal line drivingcircuit 601.

The shift register 602 generates timing signals in response to clocksignals CLK and start pulse signals SP inputted from given wiring lines.The timing signals are respectively inputted to a plurality of latches A(LATA_1 to LATA_x) of the memory circuit A 603. The timing signalsgenerated in the shift register 602 may be buffered and amplified by abuffer or the like before inputting the signals to the plural latches A(LATA_1 to LATA_x) of the memory circuit A 603.

When timing signals are inputted to the memory circuit A 603, in syncwith the timing signals, digital video signals equivalent to one bitwhich are inputted to a video signal line 610 are sequentially writtenin the plural latches A (LATA_1 to LATA_x) to be stored therein.

In this embodiment, digital video signals are sequentially inputted tothe plural latches A (LATA_1 to LATA_x) of the memory circuit A 603 wheninputting digital video signals into the memory circuit A 603. However,the present invention is not limited thereto. The invention may employ aso-called division driving in which the plural stages of lathes of thememory circuit A 603 are divided into a few groups and digital videosignals are inputted to the respective groups simultaneously. The numberof groups in division driving is referred to as number of division. Forexample, if four stages of latches make one group, then it is fourdivision driving.

The time required for completing writing digital video signals once intoall stages of latches of the memory circuit A 603 is called a lineperiod. In practice, sometimes the line period defined as above plus ahorizontal retrace period are regarded as a line period.

Upon completion of one line period, latch signals are supplied to aplurality of latches B (LATB_1 to LATB_x) of the memory circuit B 604through a latch signal line 609. At this instant, the digital videosignals that have been held in the plural latches A (LATA_1 to LATA_x)of the memory circuit A 603 are sent to the plural latches B (LATB_1 toLATB_x) of the memory circuit B 604 all at once to be written and heldtherein.

Having sent the digital video signals to the memory circuit B 604, thememory circuit A 603 now receives the next supply of digital videosignals equivalent to one bit so that the digital video signals aresequentially written in response to timing signals from the shiftregister 602.

After one line period is thus started for the second time, the digitalvideo signals written and held in the memory circuit B 604 are inputtedto the constant current circuit 605.

The constant current circuit 605 has a plurality of current settingcircuits (C1 to Cx). When digital video signals are respectivelyinputted to the current setting circuits (C1 to Cx), information of ‘0’or ‘1’ contained in the digital video signals determines whether aconstant current Ic flows in the signal line or the signal line receivesthe electric potential of power supply lines V1 to Vx.

FIG. 19 shows an example of the specific structure for the currentsetting circuit C1. This structure is shared by the current settingcircuits C2 to Cx.

The current setting circuit C1 has a constant current source 631, fourtransmission gates SW1 to SW4, and two inverters Inb1 and Inb2. Atransistor 650 of the constant current source 631 has the same polarityas those of transistors Tr1 and Tr2 of each pixel.

Digital video signals outputted from the LATB_1 of the memory circuit B604 are used to control switching of SW1 to SW4. Digital video signalsinputted to SW1 and SW3 are inverted by Inb1 and Inb2 and the inverteddigital video signals are inputted to SW2 and SW4. Accordingly, SW2 andSW4 are OFF when SW1 and SW3 are ON and, when SW1 and SW3 are OFF, SW2and SW4 are ON.

When SW1 and SW3 are ON, the current Ic having a given value other than0 is inputted from the constant current source 631 to a signal line S1through SW1 and SW3.

On the other hand, when SW2 and SW4 are ON, the current Ic from theconstant current source 631 is dropped to the ground through SW2 and thepower supply electric potential of the power supply lines V1 to Vx isgiven to the signal line S1 to set Ic nearly equal to 0 through SW4.

Back to FIG. 18, the operation described above is simultaneouslyconducted in all of the current setting circuits (C1 to Cx) of theconstant current circuit 605 in one line period. Therefore the value ofthe signal current Ic to be inputted is determined for the respectivesignal lines by digital video signals.

The structure of the scanning line driving circuit is described next.

FIG. 20 is a block diagram showing the structure of a scanning linedriving circuit 641.

The scanning line driving circuit 641 has a shift register 642 and abuffer 643. In some cases, the scanning line driving circuit may have alevel shifter.

In the scanning line driving circuit 641, timing signals are generatedupon input of clock signals CLK and start pulse signals SP to the shiftregister 642. The timing signals generated are buffered and amplified bythe buffer 643 and then the signals are supplied to associated scanninglines.

One scanning line is connected to gate electrodes of first switchingtransistors and second switching transistors of one line of pixels.Since the first switching transistors and second switching transistorsof one line of pixels have to be turned ON all at once, the buffer 643used is capable of causing a large amount of current to flow.

Structures of the driving circuits used in the present invention are notlimited to those shown in this embodiment. The structure of the constantcurrent circuit of this embodiment is not limited to the one illustratedin FIG. 19. The constant current circuit used in the present inventioncan have any structure as long as it can cause the signal current Icwhose value is chosen from two values by a digital video signal to flowinto a signal line.

The structure of this embodiment can be combined freely with Embodiments1 through 5.

Embodiment 7

This embodiment describes an order sub-frame periods SF1 to SFn turn upin a method of driving a light emitting device of the present inventionfor n bit digital video signals.

FIG. 21 shows at which points n writing periods (Ta1 to Tan) and ndisplay periods (Td1 to Tdn) are started in one frame period. The axisof abscissa indicates time whereas the axis of ordinate indicatespositions of scanning lines of pixels. Descriptions on details about howthe pixels operate are omitted here but can be found in EmbodimentModes.

According to the driving method of this embodiment, the sub-frame periodthat has the longest display period in one frame period (in thisembodiment. SFn) does not come first or last in the one frame period. Inother words, the sub-frame period that has the longest display period inone frame period is interposed between other sub-frame periods of thesame frame period.

This makes it difficult for the human eye to recognize uneven displaycaused by light emission in close display periods in adjacent frameperiods when an image is displayed with intermediate gray scales.

The structure of this embodiment is effective when n≧3. This embodimentmay be combined freely with Embodiments 1 through 6.

Embodiment 8

This embodiment describes a case of driving a light emitting device ofthe present invention using 6 bit digital video signals.

FIG. 22 shows at which points six writing periods (Ta1 to Ta6) and sixdisplay periods (Td1 to Td6) are started in one frame period. The axisof abscissa indicates time whereas the axis of ordinate indicatespositions of scanning lines of pixels. Descriptions on details about howpixels operate are omitted here but can be found in Embodiment Modes.

When the light emitting device is driven using 6 bit digital videosignals, at least six sub-frame periods SF1 to SF6 are provided in oneframe period.

The sub-frame periods SF1 to SF6 are respectively associated with 1 bitdigital video signals to 6 bit digital signals. The sub-frame periodsSF1 to SF6 have six writing periods (Ta1 to Ta6) and six display periods(Td1 to Td6).

A sub-frame period SFm (m is an arbitrary number out of 1 through 6) hasa writing period Tam and a display period Tdm that are associated withthe m-th bit digital video signals. The writing period Tam is followedby a display period that is associated with the same bit number, in thiscase, the display period Tdm.

A writing period Ta and a display period Td are repeatedly alternated inone frame period to make it possible to display one image.

Lengths of the display periods Td1 to Td6 are set to satisfy Td1:Td2: .. . :Td6=2⁰:2¹: . . . :2⁵.

In the driving method according to the present invention, gray scalesare obtained by controlling the sum of lengths of display periods in oneframe period in which light is emitted.

The structure of this embodiment may be combined freely with Embodiments1 through 7.

Embodiment 9

This embodiment describes a driving method using n bit digital videosignals which is different from those illustrated in FIGS. 6 and 21.

FIG. 23 shows at which points n+1 writing periods (Ta1 to Ta (n+1)) andn+1 display periods (Td1 to Td (n+1)) are started in one frame period.The axis of abscissa indicates time whereas the axis of ordinateindicates positions of scanning lines of pixels. Descriptions on detailsabout how the pixels operate are omitted here but can be found inEmbodiment Modes.

In this embodiment, one frame period has n+1 sub-frame periods SF1 to SF(n+1) in accordance with n bit digital video signals. The sub-frameperiods SF1 to SF (n+1) have n+1 writing periods (Ta1 to Ta (n+1)) andn+1 display periods (Td1 to Td (n+1)).

A writing period Tam (m is an arbitrary number ranging from L to n+1)and a display period Tdm make a sub-frame period SFm. The writing periodTam is followed by a display period associated with the same bit number,in this case, the display period Tdm.

The sub-frame periods SF1 to SF (n−1) are respectively associated with 1bit digital video signals to (n−1) bit digital video signals. Thesub-frame periods SFn and SF (n+1) are both associated with the n-th bitdigital video signals.

The sub-frame periods SFn and SF (n+1) that are for the same bit numberdo not immediately follow each other in this embodiment. In other words,the sub-frame periods SFn and SF (n+1) that are for digital videosignals of the same bit number sandwich another sub-frame period.

A writing period Ta and a display period Td are repeatedly alternated inone frame period to make it possible to display one image.

Lengths of the display periods Td1 to Td (n+1) are set so as to satisfyTd1:Td2: . . . :(Tdn+Td(n+1))=2⁰:2¹: . . . :2^(n-1).

According to the driving method of the present invention, gray scaledisplay is obtained by controlling the total light emission time of apixel in one frame period, namely, for how many display periods in oneframe period the pixel emits light.

The above structure makes the uneven display in intermediate gray scaledisplay less recognizable to the human eye than in the cases illustratedin FIGS. 6 and 21. The uneven display is caused by adjoining displayperiods during which light is emitted in adjacent frame periods.

Described in this embodiment is the case in which two sub-frame periodsare provided for digital video signal of the same bit. However, thepresent invention is not limited thereto. Three or more sub-frameperiods may be provided for the same bit in one frame period.

Although a plurality of sub-frame periods are provided for the mostsignificant bit digital video signal in this embodiment, the presentinvention is not limited thereto. A digital video signal of other bitthan the most significant bit may have a plurality of sub-frame periods.There is no need to limit the number of digital video signal bits thatcan have a plurality of sub-frame periods to one. A digital video signalof certain bit and a digital video signal of another bit canrespectively have plural sub-frame periods.

The structure of this embodiment is effective when n≧2. This embodimentcan be combined freely with Embodiments 1 through 8.

Embodiment 10

This embodiment describes the structure of a signal line driving circuitin a light emitting device of the present invention when the device isdriven by an analog driving method. A scanning line driving circuit ofthis device can have the structure shown in Embodiment 6 and thedescription thereof is omitted here.

FIG. 31A is a block diagram of a signal line driving circuit 401 of thisembodiment. Reference symbol 402 denotes a shift register, 403, abuffer, 404, a sampling circuit, and 405, a current converting circuit.

Clock signals (CLK) and start pulse signals (SP) are inputted to theshift register 402. The shift register 402 generates timing signals inresponse to input of clock signals (CLK) and start pulse signals (SP).

The timing signals generated are amplified, or buffered and amplified,by the buffer 403 before inputted to the sampling circuit 404. Thetiming signals may be amplified by a level shifter instead of a buffer.Alternatively, the driving circuit may have a buffer and a level shifterboth.

FIG. 31B shows specific structures of the sampling circuit 404 and thecurrent converting circuit 405. The sampling circuit 404 is connected tothe buffer 403 at a terminal 410.

The sampling circuit 404 is provided with a plurality of switches 411.Analog video signals are inputted from video signal lines 406 to thesampling circuit 404. The switches 411 sample the analog video signalsin sync with the timing signals and then input the sampled signals tothe current converting circuit 405 downstream thereof. The only currentconverting circuit 405 shown in FIG. 31B is one that is connected to oneof the switches 411 of the sampling circuit 404. However, currentconverting circuits identical with the current converting circuit 405shown in FIG. 31B are connected downstream of the respective switches411.

Each of the switches 411 is composed of one transistor in thisembodiment. However, the structure of the switches 411 is not limited tothe one shown in this embodiment and any switch can be used as long asit can sample analog video signals in sync with timing signals.

The analog video signals sampled are inputted to a current outputcircuit 412 of the current converting circuit 405. The current outputcircuit 412 outputs a current (signal current) in an amount according tothe voltage of the inputted video signals. Although the current outputcircuit in FIG. 31B is composed of an amplifier and a transistor, thepresent invention is not limited thereto. The current output circuit maybe any circuit as long as it can output a current in an amount accordingto the voltage of a signal inputted.

The signal current is inputted to a reset circuit 417 within the samecurrent converting circuit 405. The reset circuit 417 has two analogswitches 413 and 414, an inverter 416, and a power supply 415.

Reset signals (Res) are inputted to the analog switch 414. Reset signals(Res) inverted by the inverter 416 are inputted to the analog switch413. The analog switch 413 and the analog switch 414 operate in syncwith inverted reset signals and reset signals, respectively, andtherefore one is ON while the other is OFF.

When the analog switch 413 is ON, the signal current is inputted to theassociated signal line. When the analog switch 414 is ON, on the otherhand, the electric potential of the power supply 415 is given to thesignal line to reset the signal line. The electric potential of thepower supply 415 is desirably at almost the same level as that of theelectric potential of a power supply line provided in a pixel. Thecloser the current flowing in a signal line during reset of the signalline to 0, the better.

It is desirable to reset a signal line during a retrace period. However,if necessary, a signal line may be reset during a period other than aretrace period except when an image is displayed.

The signal line driving circuit and scanning line driving circuit fordriving the light emitting device of the present invention are notlimited to the structures shown in this embodiment. The structure ofthis embodiment can be combined freely with the structures ofEmbodiments 1 through 9.

Embodiment 11

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an organic light emitting material by whichphosphorescence from a triplet exciton can be employed for emitting alight. As a result, the power consumption of the OLED can be reduced,the lifetime of the OLED can be elongated and the weight of the OLED canbe lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi. S. Saito. Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an organic light emitting material (coumarinpigment) reported by the above article is represented as follows.

(M. A. Baldo, D. F. O'Brien. Y. You. A. Shoustikov, S. Sibley. M. E.Thompson. S. R. Forrest. Nature 395 (1998) p. 151)

The molecular formula of an organic light emitting material (Pt complex)reported by the above article is represented as follows.

(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest.Appl. Phys. Lett., 75 (1999) p. 4.)(T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe. T. Tsuji,Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38 (12B) (1999)L1502.)

The molecular formula of an organic light emitting material (Ir complex)reported by the above article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle.

The structure according to this embodiment can be freely implemented incombination of any structures of the Embodiments 1 to 10.

Embodiment 12

In this embodiment, an example of manufacturing the light emittingdevice using the present invention is described with reference to FIGS.24 (A) to 24(C).

FIG. 24 is a top view of the light emitting device which is formedaccording as the element substrate with the transistor is sealed bysealing materials. FIG. 24 (B) is a cross sectional view taken alongwith a line A-A′ of FIG. 24 (A), and FIG. 24 (C) is a cross sectionalview taken along with a line B-B′ of FIG. 24 (A).

A seal member 4009 is provided so as to surround a pixel portion 4002, asignal line driver circuit 4003, and the first, second scanning linedriver circuits 4004 a, 4004 b, which are provided on a substrate 4001.Further, a sealing material 4008 is provided on the pixel portion 4002,the signal line driver circuit 4003, and the first, the second scanningline driver circuits 4004 a, 4004 b. Thus, the pixel portion 4002, thesignal line driver circuit 4003, and the first, the second scanning linedriver circuits 4004 a, 4004 b are sealed by the substrate 4001, theseal member 4009 and the seating material 4008 together with a filler4210.

Further, the pixel portion 4002, the signal line driver circuit 4003,and the first, the second scanning line driver circuits 4004 a, 4004 b,which are provided on the substrate 4001, have a plurality of TFTs. InFIG. 24 (B), a driver circuit TFT (Here, an n-channel TFT and ap-channel TFT are shown in the figure.) 4201 included in the signal linedriver circuit 4003 and a current controlling TFT (transistor Tr3) 4202included in the pixel portion 4002, which are formed on a base film4010, are typically shown.

In this embodiment, the p-channel TFT or the n-channel TFT manufacturedby a known method is used as the driver circuit TFT 4201, and thep-channel TFT manufactured by a known method is used as the currentcontrolling TFT 4202. Further, the display pixel portion 4002 isprovided with a storage capacitor (not shown) connected to a gateelectrode of the current controlling TFT 4202.

An interlayer insulating film (leveling film) 4301 is formed on thedriver circuit TFT 4201 and the current controlling TFT 4202, and apixel electrode (anode) 4203 electrically connected to a drain of thecurrent controlling TFT 4202 is formed thereon. A transparent conductivefilm having a large work function is used for the pixel electrode 4203.A compound of indium oxide and tin oxide, a compound of indium oxide andzinc oxide, zinc oxide, tin oxide or indium oxide can be used for thetransparent conductive film. The above transparent conductive film addedwith gallium may also be used.

Then, an insulating film 4302 is formed on the pixel electrode 4203, andthe insulating film 4302 is formed with an opening portion on the pixelelectrode 4203. In this opening portion, an organic light emitting layer4204 is formed on the pixel electrode 4203. A known organic lightemitting material or inorganic light emitting material may be used forthe organic light emitting layer 4204. Further, there exist a lowmolecular weight (monomer) material and a high molecular weight(polymer) material as the organic light emitting materials, and both thematerials may be used.

A known evaporation technique or application technique may be used as amethod of forming the organic light emitting layer 4204. Further, thestructure of the organic light emitting layer may take a laminationstructure or a single layer structure by freely combining a holeinjecting layer, a hole transporting layer, a light emitting layer, anelectron transporting layer and an electron injecting layer.

A cathode 4205 made of a conductive film having light shielding property(typically, conductive film containing aluminum, copper or silver as itsmain constituent or lamination film of the above conductive film andanother conductive film) is formed on the organic light emitting layer4204. Further, it is desirable that moisture and oxygen that exist on aninterface of the cathode 4205 and the organic light emitting layer 4204are removed as much as possible. Therefore, such a device is necessarythat the organic light emitting layer 4204 is formed in a nitrogen orrare gas atmosphere, and then, the cathode 4205 is formed withoutexposure to oxygen and moisture. In this embodiment, the above-describedfilm deposition is enabled by using a multi-chamber type (cluster tooltype) film forming device. In addition, a predetermined voltage is givento the cathode 4205.

As described above, an OLED 4303 constituted of the pixel electrode(anode) 4203, the organic light emitting layer 4204 and the cathode 4205is formed. Further, a protective film 4303 is formed on the insulatingfilm 4302 so as to cover the OLED 4303. The protective film 4209 iseffective in preventing oxygen, moisture and the like from permeatingthe OLED 4303.

Reference numeral 4005 a denotes a wiring drawn to be connected to thepower supply line, and the wiring 4005 a is electrically connected to asource region of the current controlling TFT 4202. The drawn wiring 4005a passes between the seal member 4009 and the substrate 4001, and iselectrically connected to an FPC wiring 4301 of an FPC 4006 through ananisotropic conductive film 4300.

A glass material, a metal material (typically, stainless material), aceramics material or a plastic material (including a plastic film) canbe used for the sealing material 4008. As the plastic material, an FRP(fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film or an acrylic resin film may be used.Further, a sheet with a structure in which an aluminum foil issandwiched with the PVF film or the Mylar film can also be used.

However, in the case where the light from the OLED is emitted toward thecover member side, the cover member needs to be transparent. In thiscase, a transparent substance such as a glass plate, a plastic plate, apolyester film or an acrylic film is used.

Further, in addition to an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin may be used as thefiller 4210, so that PVC (polyvinyl chloride), acrylic, polyimide, epoxyresin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylene vinylacetate) can be used. In this embodiment, nitrogen is used for thefiller.

Moreover, a concave portion 4007 is provided on the surface of thesealing material 4008 on the substrate 4001 side, and a hygroscopicsubstance or a substance that can absorb oxygen 4207 is arranged thereinin order that the filler 4210 is made to be exposed to the hygroscopicsubstance (preferably, barium oxide) or the substance that can absorboxygen. Then, the hygroscopic substance or the substance that can absorboxygen 4207 is held in the concave portion 4007 by a concave portioncover member 4208 such that the hygroscopic substance or the substancethat can absorb oxygen 4207 is not scattered. Note that the concaveportion cover member 4208 has a fine mesh form, and has a structure inwhich air and moisture are penetrated while the hygroscopic substance orthe substance that can absorb oxygen 4207 is not penetrated. Thedeterioration of the OLED 4303 can be suppressed by providing thehygroscopic substance or the substance that can absorb oxygen 4207.

As shown in FIG. 24(C), the pixel electrode 4203 is formed, and at thesame time, a conductive film 4203 a is formed so as to contact the drawnwiring 4005 a.

Further, the anisotropic conductive film 4300 has conductive filler 4300a. The conductive film 4203 a on the substrate 4001 and the FPC wiring4301 on the FPC 4006 are electrically connected to each other by theconductive filler 4300 a by heat-pressing the substrate 4001 and the FPC4006.

Note that this embodiment can be implemented by being freely combinedwith Embodiments 1 to 11.

Embodiment 13

This embodiment describes an example of the structure of a pixel in alight emitting device of the present invention which is different fromthe examples illustrated in FIGS. 2, 7, and 8.

The pixel structure of this embodiment is shown in FIG. 30A. A pixelshown in FIG. 30A is denoted by 701 and has a signal line Si (one of S1to Sx), a first scanning line Gaj (one of Ga1 to Gay), a second scanningline Gbj (one of Gb1 to Gby), and a power supply line Vi (one of V1 toVx). The number of first scanning lines and the number of secondscanning lines in a pixel portion may not always match.

The pixel 701 has, at least, a transistor Tr1 (a first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), a transistor Tr6 (an erasing transistor or a sixthtransistor), an OLED 704, and a storage capacitor 705.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the first scanning line Gaj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a drain region of the transistor Tr1. The transistor Tr5 has a sourceregion and a drain region one of which is connected to the signal lineSi and the other of which is connected to a gate electrode of thetransistor Tr3.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A gate electrode of the transistor Tr6 is connected to the secondscanning line Gbj. The transistor Tr6 has a source region and a drainregion one of which is connected to the power supply line Vi and theother of which is connected to the gate electrodes of the transistor Tr1and of the transistor Tr2.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 704. The electric potential of the power supply line Vi(power supply electric potential) is kept constant. The electricpotential of an opposite electrode is also kept constant.

The transistor Tr4 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistor Tr5. However, thetransistor Tr4 and the transistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The transistor Tr6 may be an n-channel TFT or a p-channel

The storage capacitor 705 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 705is provided to maintain the voltage between the gate electrode of thetransistor Tr3 and the source region thereof (gate voltage) moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor 1 between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

FIG. 30B shows another structure for the pixel of this embodiment. Apixel shown in FIG. 30B is denoted by 711 and has a signal line Si (oneof S1 to Sx), a first scanning line Gaj (one of Ga1 to Gay), a secondscanning line Gbj tone of Gb1 to Gby), and a power supply line Vi (oneof V1 to Vx). The number of first scanning lines and the number ofsecond scanning lines in a pixel portion may not always match.

The pixel 711 has, at least, a transistor Tr1 (a first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), a transistor Tr6 (an erasing transistor or a sixthtransistor), an OLED 714, and a storage capacitor 715.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the first scanning line Gaj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a drain region of the transistor Tr1. The transistor Tr5 has a sourceregion and a drain region one of which is connected to the drain regionof the transistor Tr1 and the other of which is connected to a gateelectrode of the transistor Tr3.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A gate electrode of the transistor Tr6 is connected to the secondscanning line Gbj. The transistor Tr6 has a source region and a drainregion one of which is connected to the power supply line Vi and theother of which is connected to the gate electrodes of the transistor Tr1and of the transistor Tr2.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 714. The electric potential of the power supply line Vi(power supply electric potential) is kept constant. The electricpotential of an opposite electrode is also kept constant.

The transistor Tr4 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistor Tr5. However, thetransistor Tr4 and the transistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The transistor Tr6 may be an n-channel transistor or a p-channeltransistor.

The storage capacitor 715 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 715is provided to maintain the gate voltage of the transistor Tr3 moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor a between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

FIG. 30C shows another structure for the pixel of this embodiment. Apixel shown in FIG. 30C is denoted by 721 and has a signal line Si (oneof S1 to Sx), a first scanning line Gaj (one of Ga1 to Gay), a secondscanning line Gbj (one of Gb1 to Gby), and a power supply line Vi (oneof V1 to Vx). The number of first scanning lines and the number ofsecond scanning lines in a pixel portion may not always match.

The pixel 721 has, at least, a transistor Tr1 (a first currentcontrolling transistor or a first transistor), a transistor Tr2 (asecond current controlling transistor or a second transistor), atransistor Tr3 (a third current controlling transistor or a thirdtransistor), a transistor Tr4 (a first switching transistor or a fourthtransistor), a transistor Tr5 (a second switching transistor or a fifthtransistor), a transistor Tr6 (an erasing transistor or a sixthtransistor), an OLED 724, and a storage capacitor 725.

Gate electrodes of the transistor Tr4 and of the transistor Tr5 areconnected to the first scanning line Gaj.

The transistor Tr4 has a source region and a drain region one of whichis connected to the signal line Si and the other of which is connectedto a gate electrode of the transistor Tr3. The transistor Tr5 has asource region and a drain region one of which is connected to the gateelectrode of the transistor Tr3 and the other of which is connected to adrain region of the transistor Tr1.

Gate electrodes of the transistor Tr1 and of the transistor Tr2 areconnected to each other. Source regions of the transistor Tr1 and of thetransistor Tr2 are connected to the power supply line Vi.

The gate electrode of the transistor Tr2 is connected to a drain regionthereof. The drain region of the transistor Tr2 is connected to a sourceregion of the transistor Tr3.

A gate electrode of the transistor Tr6 is connected to the secondscanning line Gbj. The transistor Tr6 has a source region and a drainregion one of which is connected to the power supply line Vi and theother of which is connected to the gate electrodes of the transistor Tr1and of the transistor Tr2.

A drain region of the transistor Tr3 is connected to a pixel electrodeof the OLED 724. The electric potential of the power supply line Vi(power supply electric potential) is kept constant. The electricpotential of an opposite electrode is also kept constant.

The transistor Tr4 may be an n-channel TFT or a p-channel TFT and thesame applies to the transistor Tr5. However, the transistor Tr4 and thetransistor Tr5 have to have the same polarity.

The transistor Tr1 may be an n-channel transistor or a p-channeltransistor and the same applies to the transistors Tr2 and Tr3. However,the transistors Tr1, Tr2, and Tr3 have to have the same polarity. Whenthe pixel electrode serves as an anode and the opposite electrode servesas a cathode, the transistors Tr1, Tr2, and Tr3 are p-channeltransistors. On the other hand, n-channel transistors are used for thetransistors Tr1, Tr2, and Tr3 when the opposite electrode serves as ananode and the pixel electrode serves as a cathode.

The transistor Tr6 may be an n-channel transistor or a p-channeltransistor.

The storage capacitor 725 is formed between the gate electrode of thetransistor Tr3 and the power supply line Vi. The storage capacitor 725is provided to maintain the voltage between the gate electrode of thetransistor Tr3 and the source region thereof (gate voltage) moresecurely but it may not always be necessary.

The transistors Tr1 and Tr2 may have storage capacitor s between theirgate electrodes and the power supply line so that the gate voltages ofthe transistors Tr1 and Tr2 can be maintained more securely.

A light emitting device having a pixel structured as shown in FIGS. 30A.30B or 30C is driven by a digital driving method and an analog drivingmethod cannot be used to drive the device. In the pixels shown in FIGS.30A, 30B, and 30C, it is possible to make the OLEDs 704, 714, and 724stop emitting light by controlling the electric potential of the secondscanning line Gbj so as to turn the transistor Tr5 ON while the OLEDsare emitting light. Therefore display periods of pixels can be forcedlyterminated while inputting digital video signals to pixels. Displayperiods thus can be made shorter than writing periods and the pixelstructures are suitable for driving the device using digital videosignals of high bit number.

The structure of this embodiment may be combined freely with thestructures shown in Embodiments 1, 2, 5, 6, 7, 8, 9, 11, and 12.

Embodiment 14

The light emitting device using the OLED is of the self-emission type,and thus exhibits more excellent recognizability of the displayed imagein a light place as compared to the liquid crystal display device.Furthermore, the light emitting device has a wider viewing angle.Accordingly, the light emitting device can be applied to a displayportion in various electronic devices.

Such electronic devices using a light emitting device of the presentinvention include a video camera, a digital camera, a goggles-typedisplay (head mount display), a navigation system, a sound reproductiondevice (a car audio equipment and an audio set), a lap-top computer, agame machine, a portable information terminal (a mobile computer, aportable telephone, a portable game machine, an electronic book, or thelike), an image reproduction apparatus including a recording medium(more specifically, an apparatus which can reproduce a recording mediumsuch as a digital video disc (DVD) and so forth, and includes a displayfor displaying the reproduced image), or the like. In particular, in thecase of the portable information terminal, use of the light emittingdevice is preferable, since the portable information terminal that islikely to be viewed from a tilted direction is often required to have awide viewing angle. FIG. 25A to 25H respectively shows various specificexamples of such electronic equipment.

FIG. 25 (A) illustrates a display device using OLED which includes acasing 2001, a support table 2002, a display portion 2003, a speakerportion 2004, a video input terminal 2005 or the like. The presentinvention is applicable to the display portion 2003. The light emittingdevice is of the self-emission type and therefore requires no backlight. Thus, the display portion thereof can have a thickness thinnerthan that of the liquid crystal display device. The OLED display deviceis including the entire display device for displaying information, suchas a personal computer, a receiver of TV broadcasting and an advertisingdisplay.

FIG. 25 (B) illustrated a digital still camera which includes a mainbody 2101, a display portion 2102, an image receiving portion 2103, anoperation key 2104, an external connection port 2105, a shutter 2106, orthe like. The light emitting device in accordance with the presentinvention can be used as the display portion 2102.

FIG. 25 (C) illustrates a lap-top computer which includes a main body2201, a casing 2202, a display portion 2203, a keyboard 2204, anexternal connection port 2205, a pointing mouse 2206, or the like. Thelight emitting device in accordance with the present invention can beused as the display portion 2203.

FIG. 25 (D) illustrated a mobile computer which includes a main body2301, a display portion 2302, a switch 2303, an operation key 2304, aninfrared port 2305, or the like. The light emitting device in accordancewith the present invention can be used as the display portion 2302.

FIG. 25 (E) illustrates a portable image reproduction apparatusincluding a recording medium (more specifically, a DVD reproductionapparatus), which includes a main body 2401, a casing 2402, a displayportion A 2403, another display portion B 2404, a recording medium (DVDor the like) reading portion 2405, an operation key 2406, a speakerportion 2407 or the like. The display portion A 2403 is used mainly fordisplaying image information, while the display portion B 2404 is usedmainly for displaying character information. The light emitting devicein accordance with the present invention can be used as these displayportions A 2403 and B 2404. The image reproduction apparatus including arecording medium further includes a game machine or the like.

FIG. 25 (F) illustrates a goggle type display (head mounted display)which includes a main body 2501, a display portion 2502, arm portion2503 or the like. The light emitting device in accordance with thepresent invention can be used as the display portion 2502.

FIG. 25 (G) illustrates a video camera which includes a main body 2601,a display portion 2602, a casing 2603, an external connecting port 2604,a remote control receiving portion 2605, an image receiving portion2606, a battery 2607, a sound input portion 2608, an operation key 2609,or the like. The light emitting device in accordance with the presentinvention can be used as the display portion 2602.

FIG. 25 (H) illustrates a portable telephone which includes a main body2701, a casing 2702, a display portion 2703, a sound input portion 2704,a sound output portion 2705, an operation key 2706, an externalconnecting port 2707, an antenna 2708, or the like. The light emittingdevice in accordance with the present invention can be used as thedisplay portion 2703. Note that the display portion 2703 can reducepower consumption of the portable telephone by displaying white-coloredcharacters on a black-colored background.

When the brighter luminance of light emitted from the organic lightemitting material becomes available in the future, the light emittingdevice in accordance with the present invention will be applicable to afront-type or rear-type projector in which light including output imageinformation is enlarged by means of lenses or the like to be projected.

The aforementioned electronic equipments are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. The light emitting device issuitable for displaying moving pictures since the organic light emittingmaterial can exhibit high response speed.

A portion of the light emitting device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light emitting portion therein becomes as small as possible.Accordingly, when the light emitting device is applied to a displayportion which mainly displays character information, e.g. a displayportion of a portable information terminal, and more particular, aportable telephone or a sound reproduction device, it is desirable todrive the light emitting device so that the character information isformed by a light emitting portion while a non-emission portioncorresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic devices in all fields. The electronic device inthis embodiment can be obtained by utilizing a light emitting devicehaving the configuration in which the structures in Embodiments 1 to 13are freely combined.

With the structure described above, a light emitting device of thepresent invention can keep the luminance constant without beinginfluenced by temperature change. If the device is to display an imagein color and different organic light emitting materials are used forOLEDs of different colors, the luminance of the OLEDs of differentcolors changes uniformly to obtain desired colors.

1. A display device comprising: a pixel formed over a substrate, thepixel comprising a light emitting element, a first transistor, a secondtransistor, a third transistor, a fourth transistor and a capacitor; agate driver circuit formed over the substrate; a terminal formed overthe substrate; and a wiring formed over the substrate, the wiringelectrically connected to the terminal, wherein the light emittingelement comprises a pixel electrode, an organic compound layer over thepixel electrode and an opposite electrode over the organic compoundlayer, wherein the wiring is in direct contact with the oppositeelectrode, wherein the first transistor and the second transistor areelectrically connected in series between the pixel electrode of thelight emitting element and a first line, wherein one of a source and adrain of the third transistor is electrically connected to a gate of thefirst transistor, wherein one of a source and a drain of the fourthtransistor is electrically connected to the other of the source and thedrain of the third transistor, wherein the other of the source and thedrain of the fourth transistor is electrically connected to a currentsource circuit via a second line, wherein a gate of the fourthtransistor is electrically connected to a third line, wherein a firstelectrode of the capacitor is electrically connected to the gate of thefirst transistor, and wherein a second electrode of the capacitor iselectrically connected to the first line.
 2. The display deviceaccording to claim 1, further comprising a fifth transistor, wherein oneof a source and a drain of the fifth transistor is electricallyconnected to a gate of the second transistor, wherein the other of thesource and the drain of the fifth transistor is electrically connectedto the first line, and wherein a gate of the fifth transistor iselectrically connected to a fourth line.
 3. The display device accordingto claim 2, further comprising a sixth transistor, wherein a gate of thesixth transistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the sixth transistoris electrically connected to the first line, and wherein the other ofthe source and the drain of the sixth transistor is electricallyconnected to the other of the source and the drain of the thirdtransistor.
 4. The display device according to claim 1, wherein one of asource and a drain of the second transistor is electrically connected tothe first line, wherein the other of the source and the drain of thesecond transistor is electrically connected to one of a source and adrain of the first transistor, and wherein the other of the source andthe drain of the first transistor is electrically connected to the lightemitting element.
 5. The display device according to claim 4, whereinthe gate of the second transistor is electrically connected to the otherof the source and the drain of the second transistor.
 6. The displaydevice according to claim 1, wherein the wiring and the third line areformed on a same layer.
 7. The display device according to claim 1,wherein a gate of the third transistor is electrically connected to thegate of the fourth transistor.
 8. The display device according to claim1, wherein the first transistor comprises polycrystalline silicon. 9.The display device according to claim 1, wherein the light emittingelement is an organic light emitting diode.
 10. The display deviceaccording to claim 1, wherein the first line is a power supply line. 11.A display device comprising: a pixel formed over a substrate, the pixelcomprising a light emitting element, a first transistor, a secondtransistor, a third transistor, a fourth transistor and a capacitor; agate driver circuit formed over the substrate; a terminal formed overthe substrate; and a wiring formed over the substrate, the wiringelectrically connected to the terminal, wherein the first transistor andthe second transistor are electrically connected in series between thelight emitting element and a first line, wherein one of a source and adrain of the third transistor is electrically connected to a gate of thefirst transistor, wherein one of a source and a drain of the fourthtransistor is electrically connected to the other of the source and thedrain of the third transistor, wherein the other of the source and thedrain of the fourth transistor is electrically connected to a currentsource circuit via a second line, wherein a gate of the fourthtransistor is electrically connected to a third line, wherein a firstelectrode of the capacitor is electrically connected to the gate of thefirst transistor, wherein a second electrode of the capacitor iselectrically connected to the first line, and wherein the secondtransistor is turned on and off so as to be turned on more than once inone frame period.
 12. The display device according to claim 11, furthercomprising a fifth transistor, wherein one of a source and a drain ofthe fifth transistor is electrically connected to a gate of the secondtransistor, wherein the other of the source and the drain of the fifthtransistor is electrically connected to the first line, wherein a gateof the fifth transistor is electrically connected to a fourth line, andwherein the second transistor is turned off by inputting a signal to thefourth line.
 13. The display device according to claim 12, furthercomprising a sixth transistor, wherein a gate of the sixth transistor iselectrically connected to the gate of the second transistor, wherein oneof a source and a drain of the sixth transistor is electricallyconnected to the first line, and wherein the other of the source and thedrain of the sixth transistor is electrically connected to the other ofthe source and the drain of the third transistor.
 14. The display deviceaccording to claim 11, wherein one of a source and a drain of the secondtransistor is electrically connected to the first line, wherein theother of the source and the drain of the second transistor iselectrically connected to one of a source and a drain of the firsttransistor, and wherein the other of the source and the drain of thefirst transistor is electrically connected to the light emittingelement.
 15. The display device according to claim 14, wherein the gateof the second transistor is electrically connected to the other of thesource and the drain of the second transistor.
 16. The display deviceaccording to claim 11, wherein the wiring and the third line are formedon a same layer.
 17. The display device according to claim 11, wherein agate of the third transistor is electrically connected to the gate ofthe fourth transistor.
 18. The display device according to claim 11,wherein the first transistor comprises polycrystalline silicon.
 19. Thedisplay device according to claim 11, wherein the light emitting elementis an organic light emitting diode.
 20. The display device according toclaim 11, wherein the first line is a power supply line.
 21. A displaydevice comprising: a light emitting element; a capacitor; a firsttransistor; a second transistor; a third transistor; and a fourthtransistor, wherein one of a source and a drain of the first transistoris electrically connected to the light emitting element, wherein theother of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the secondtransistor, wherein the other of the source and the drain of the secondtransistor is electrically connected to a first line, wherein one of asource and a drain of the third transistor is electrically connected toa gate of the first transistor, wherein the other of the source and thedrain of the third transistor is electrically connected to a secondline, wherein a gate of the third transistor is electrically connectedto a third line, wherein one of a source and a drain of the fourthtransistor is electrically connected to the other of the source and thedrain of the first transistor, wherein a first electrode of thecapacitor is electrically connected to the gate of the first transistor,and wherein a second electrode of the capacitor is electricallyconnected to the first line.
 22. The display device according to claim21, wherein the one of the source and the drain of the first transistoris directly connected to the light emitting element.
 23. The displaydevice according to claim 21, wherein the other of the source and thedrain of the fourth transistor is electrically connected to the firstline.
 24. The display device according to claim 21, wherein the one ofthe source and the drain of the second transistor is electricallyconnected to a gate of the second transistor.
 25. The display deviceaccording to claim 21, wherein the second line is supplied with a signalcurrent.
 26. The display device according to claim 21, wherein the firstline is a power supply line.